Dennis Brennecke Katrin Knickmeier Iwona Pawliczka Ursula Siebert Magnus Wahlberg *Editors*

# Marine Mammals

A Deep Dive into the World of Science

Marine Mammals

Dennis Brennecke • Katrin Knickmeier Iwona Pawliczka • Ursula Siebert • Magnus Wahlberg *Editors*

# **Marine Mammals**

A Deep Dive into the World of Science

*Editors* Dennis Brennecke Kieler Forschungswerkstatt, Kiel University and Leibniz Institute for Science and Mathematics Education Kiel, Germany

Iwona Pawliczka Prof. Krzysztof Skora Hel Marine Station, Faculty of Oceanography and Geography University of Gdansk Hel, Poland

Magnus Wahlberg Marine Biological Research Center University of Southern Denmark Kerteminde, Denmark

Katrin Knickmeier Kieler Forschungswerkstatt, Kiel University and Leibniz Institute for Science and Mathematics Education Kiel, Germany

Ursula Siebert Institute for Terrestrial and Aquatic Wildlife Research University of Veterinary Medicine Hanover, Foundation Büsum, Germany

Horizon 2020 Framework Programme (710708) Results incorporated in this standard have received funding from the European Union's Horizon 2020 research and innovation programme under the grant agreement No. 710708.

Cover photo: Hèloïse Hamel

ISBN 978-3-031-06835-5 ISBN 978-3-031-06836-2 (eBook) https://doi.org/10.1007/978-3-031-06836-2

© The Editors and the Authors 2023. This book is an open access publication.

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### **Preface**

Science is fun, interesting, challenging and engaging. Science makes it possible to discover the marvels of the natural world. With science, we can document the lives of seals and whales, diving to great depths in pursuit of their prey and yearly travelling vast distances between breeding and feeding grounds. Science is also what makes airplanes fy and trains run. And, in the long run, science is what makes us humans able to survive on Earth, in spite of the rapidly increasing human population and the challenges we face due to environmental degradation, climate change and pandemics.

As scientists, we are engaged and full of joy when speaking about our work. Some people call us the 'last truly religious people on the planet'. We love our work, and sincerely believe that science can change the world for the better.

In spite of all the virtues of science, we predict a shortage of scientists in many western societies in the future. We will not only need more people doing research, but also engineers, technicians, veterinarians and medical doctors. There seems to be a general lack of enthusiasm among young people in taking up the challenge of studying natural science which paves the road to an interesting, challenging and fascinating future with relatively well-paid and attractive jobs.

Why is that? The major problem may be the way we try to inspire young people in choosing their career. There is a long way from the enthusiastic 'mad scientists' you meet at universities, to the rather dry presentation of science in many secondary and high school textbooks. Even though modern science centres and media platforms are doing a great job to inspire visitors and viewers to learn about research, more tools are needed to catch the interest of the next generation in science.

Marine mammals easily arouse the interest of students. They are sentinel species in many of the current debates about environmental issues: they are affected by fsheries, plastics in the oceans, environmental pollutants, anthropogenic noise and climate change. Understanding how marine mammals are affected by human activities opens up exciting discussions around many different scientifc disciplines, such as biology, chemistry, physics and oceanography.

The book is written to be an inspiration and guide for teachers to expand their way of teaching biology and other natural sciences in secondary and high schools. The book is also intended for students to learn more about the natural world. We have collected a series of topics, all related to current research in marine mammals, with focus on species found in the Baltic and North Sea. We hope that whales and seals will work as a portal to entice students to become interested not only in marine mammals and marine biology, but also in animal conservation and nature protection, and many other scientifc topics.

The book is divided into chapters covering topics such as marine mammal biology, sound and hearing, plastics, pollutants and fsheries. Each chapter introduces the topic and gives important background facts that the teacher can use in introductory presentations. At the end of each chapter, there is a small section of current hot topics in marine mammal research that can be used to spur the interest of students when seeing themselves as future scientists with all the excitement such work brings. Finally, we suggest several teaching modules that can be directly implemented in hands-on activities, inside as well as outside the classroom. At the end of a book is a glossary, explaining many of the specialized and perhaps unfamiliar words used in the chapters. Other words, that are not explained but that we expect the reader may have to look up in e.g., Wikipedia, are indicated in italics.

This book is the result of the EU H2020-funded MARINE MAM-MALS project, which was launched in 2016 and ended in 2019. Within this project, we developed teaching materials and tried it out on teachers and students. Now is the time for you to explore the result of these efforts. We hope that teachers as well as students will fnd the material interesting, and also that teachers will develop the material according to the needs of their students. We also hope that we can challenge the teachers' thinking about teaching, and also that we can help making students more interested in science.

Each chapter has been through extensive reviews by acknowledged experts in marine mammal biology as well as experienced teachers. We thank Jay Barlow, Annalisa Berta, Rune Dietz, Peter Evans, Jamileh Javid Pour, Sara Königson, Morgan Martin, Felix Mittermayer, Paul Nachtigall, Filipa Paiva, Joseph Schnitzler, Martin Thiel and Peter Tyack for reviewing individual chapters, and Alejandro Acevedo-Gutierrez, Niels Dohn, Carl Kinze, Kristine Pape, Ulf Saure, Volker Smit, Janto Schönberg, Ewa Wink and Bernd Würsig for providing fullbook reviews.

### **Dennis Brennecke**

Kiel, Germany

**Katrin Knickmeier**  Kiel, Germany

**Iwona Pawliczka**  Hel, Poland

**Ursula Siebert**  Büsum, Germany

**Magnus Wahlberg**  Kerteminde, Denmark

### **Contents**

### **Contributors**

**Dennis Brennecke** Kieler Forschungswerkstatt, Kiel University and Leibniz Institute for Science and Mathematics Education, Kiel, Germany

**Krishna Das** Freshwater and Oceanic Sciences Unit of Research, University of Liège, Liège, Belgium

**Katrin Knickmeier** Kieler Forschungswerkstatt, Kiel University and Leibniz Institute for Science and Mathematics Education, Kiel, Germany

**Miko**ł**aj Koss** Prof. Krzysztof Skóra Hel Marine Station, Faculty of Oceanography and Geography, University of Gdańsk, Hel, Poland Foundation for the Development of the University of Gdańsk, Gdańsk, Poland

**Katrin Kruse** Kieler Forschungswerkstatt, Kiel University and Leibniz Insti-

tute for Science and Mathematics Education, Kiel, Germany

**Anna Lorenz** Institute of Aquatic Resources, Swedish University of Agricultural Sciences, Uppsala, Sweden

**Eric Parmentier** Freshwater and Oceanic Sciences Unit of Research, University of Liège, Liège, Belgium

**Iwona Pawliczka** Prof. Krzysztof Skóra Hel Marine Station, Faculty of Oceanography and Geography, University of Gdańsk, Hel, Poland

**Anja Reckendorf** Institute for Terrestrial and Aquatic Wildlife Research, University of Veterinary Medicine Hanover, Foundation, Büsum, Germany

Institute for Parasitology, University of Veterinary Medicine Hanover, Foundation, Hannover, Germany

**Lars Seidelin** Department of Mathematics and Computer Science, LSUL— Laboratory for Coherent Educations and Learning, University of Southern Denmark, Odense, Denmark

**Ursula Siebert** Institute for Terrestrial and Aquatic Wildlife Research, University of Veterinary Medicine Hanover, Foundation, Büsum, Germany Institute for Parasitology, University of Veterinary Medicine Hanover, Foundation, Hannover, Germany

**Helen Sköld** Havets Hus, Lysekil, Sweden

**Martin Stjernstedt** Havets Hus, Lysekil, Sweden

#### X Contributors

**Bianca Unger** Institute for Terrestrial and Aquatic Wildlife Research, University of Veterinary Medicine Hanover, Foundation, Büsum, Germany

**Magnus Wahlberg** Marine Biological Research Center, University of Southern Denmark, Kerteminde, Denmark Fjord&Bælt, Margrethes Plads 1, Kerteminde, Denmark

# **Who Are the Marine Mammals?**

*Krishna Das, Helen Sköld, Anna Lorenz, and Eric Parmentier*

### **Contents**

	- 5 Get acquainted with marine mammals and how they have evolved.
	- 5 Understanding physiological and morphological adaptations to marine life.
	- 5 Learn about marine mammals found in the North Sea and Baltic Sea.

### **1 Introduction**

Marine mammals are found in several lineages among the more than 6000 species of mammals (all belonging to the class Mammalia). Not all marine mammals are closely related. Together they form a habitat-based group of animals associated with water. Mammals evolved on land some 250 million years ago. At least seven separate clades of mammals have returned independently to water, where they have adapted to aquatic habitats. Despite huge differences between species, terrestrial and marine mammals share many common features (. Table 1).

Today, some 130 living species of mammals depend on the ocean for most, or all, of their lives. Marine mammals are divided into three taxonomic orders (Cetacea, Sirenia and Carnivora) with different terrestrial ancestors (. Table 2, . Figs. 1 and 2).

Cetaceans (whales) evolved from terrestrial ancestors which were hoofed animals (ungulates, belonging to the order Artiodactyla which also contain hippopotamus, pigs and deer) more than 50 million years ago, during the geological epoch called the Eocene. The closest now-living terrestrial relative of cetaceans is the hippopotamus and the ruminants, including cattle. One of the earliest known cetaceans is *Pakicetus* (meaning 'the cetacean from Pakistan'). *Pakicetus* was mostly terrestrial but walked and most probably hunted in shallow waters. It looked like a small dog with a long snout, incisors and hooves.

Sirenians (sea cows) are herbivores that emerged during the same time period as cetaceans. Sirenians share a common ancestor with elephants and hyraxes. Early sirenians possessed an elongated body and had four legs, with dense and large ribs. They were

probably found in rivers and estuaries, living from plants and seagrass.

Pinnipeds (seals, sea lions and walruses) evolved from another group of terrestrial mammals 25 million years ago during the late Oligocene epoch. Pinnipeds shares a common ancestry with so-called *arctoid carnivores*, to which both bears and otters belong.

The polar bear (from the bear family, Ursidae) and sea and marine otters (from the family Mustelidae) are more recently derived from terrestrial clades. Polar bears evolved from brown bears around 600 thousand years ago. They have adapted to a marine and Arctic lifestyle through a lack of fur pigmentation, shorter but more curved claws, specialized front paws for swimming, and a larger and more round body. Sea and marine otters have aquatic adaptations that distinguish them from other mustelids, such as their large size.

..      **Fig. 2** Different marine mammals. Not to scale. Drawings by Annika Toth

### z **Name: Harbour porpoise (**. **Fig.** 3**)**

### *Scientifc name*: *Phocoena phocoena*

*Behaviour*: Usually a rather slow swimmer, generally wary of boats, and almost never seen to bow-ride (i.e. surf the wave created by boats, as often observed in dolphins). They can speed up and change direction quickly, sometimes making arc-shaped leaps when chasing prey. Most dives last less than 5 min. The blow is not easily seen but can be heard from quite some distance in calm weather and resembles a sharp, puffing sound. Harbour porpoises are often sighted by themselves or in small groups and prefer coastal and shallow waters, even though in some areas they are also found in deeper waters. Occasionally, they have been observed swimming up rivers.

### **2 Marine mammals from the North and Baltic Seas**

The IUCN Red List of Threatened Species is a comprehensive inventory of the global conservation status of plant and animal species. It uses a set of quantitative criteria to evaluate the extinction risk of thousands of species. These criteria are relevant to most species and all regions of the world. The IUCN Red List is an authoritative guide to the status of biological diversity.

In descending order of threat, the IUCN Red List threat categories are as follows:

Extinct Extinct in the wild Critically endangered Endangered Vulnerable Near threatened Least concern Data defcient

*Distribution*: Coastal waters of the Northern Hemisphere, both in the Atlantic and Pacifc Ocean. Also found in the Baltic and Black Seas. Probably rarer in deeper waters.

*Description*: Compared to most dolphins, the rostrum of porpoises is shorter. The dorsal side is dark grey, while the ventral side is white, and there is often a dark stripe from the corner of the mouth to the pectoral fn. The dorsal fn is low and triangular. The body shape is rather short and round, limiting heat loss in cold waters.

*Weight and size*: 35–90 kg and 135–180 cm in length. Females are larger than males.

*Lifespan*: Approximately 3–10 years, rarely up to 20 years or more.

..      **Fig. 4** Harbour seal, *Phoca vitulina*. © Prof. Krzysztof Skóra Hel Marine Station, University of Gdańsk

*Diet*: Fish (e.g., herring, gobids, sprat, cod, whiting, sole, sand eel), crustaceans and squid.

*Potential predators*: Grey seals, sharks and orcas; also killed by bottlenose dolphins.

*IUCN red list status*: North Sea and inner Danish water populations are of least concern, whereas the Baltic Sea population is critically endangered.

*Special features*: In the Baltic Sea there are two spatially separated and genetically distinct populations.

### z **Name: Harbour seal (**. **Fig.** 4**)**

### *Scientifc name*: *Phoca vitulina*

*Behaviour*: Harbour seals usually swim and dive alone, whereas on land they can congregate in great numbers. They usually stay within 50 km from where they reside on land but can also cover longer distances on feeding trips, males generally swimming larger distances than females. Harbour seals haul out on land to rest, thermoregulate, and give birth. On land, they are shy and easily disturbed by nearby boats or humans.

They are less vocal in air than grey seals but sometimes produce grunting and yelping sounds. Under water, the most commonly heard sounds are from males making repetitive stereotypical calls during the mating period in the summer. Harbour seals are curious and often play by themselves with kelp or other objects.

*Distribution*: Coastal waters in the North Atlantic and North Pacifc Oceans, and in the southwestern Baltic Sea.

*Description*: Round head, pelage in different colours and with different patches. Short, dog-shaped snout with V-shaped nostrils.

*Weight and size*: Adult males are 160– 190 cm long and weigh 70–150 kg, adult females are 150–170 cm long and weigh 60–110 kg. Size may vary between populations.

*Lifespan*: Up to 20 years, with females living longer than males.

*Diet*: Generalist feeder with a varied diet consisting of fsh, cephalopods, and crustaceans. Diet varies between populations and can be area- and season-specifc.

*Natural predators*: Orcas, sharks, walruses, grey seals and eagles (pups).

*IUCN red list status*: Least concern.

*Special features*: One of the most widely distributed species of pinnipeds. In the southern Swedish part of the Baltic Sea, there is a small and genetically distinct population.

### z **Name: Grey seal (**. **Fig.** 5**)**

*Scientifc name*: *Halichoerus grypus*

*Behaviour*: Grey seals may forage far offshore and spend the remainder of their time close to the coast. They are opportunistic feeders, consuming four to six percent of their body weight per day. Feeding methods vary between areas and populations. Small fsh are usually consumed underwater and swallowed whole. Large fsh are brought to the surface and held by prehensile front fippers. The fsh head is bitten off and discarded, while the remainder of the fsh is broken into small pieces and swallowed.

*Distribution*: East and western Atlantic, also in the Baltic Sea.

 W-shaped nostrils well separated at base. Females are silver-grey with dark patches on the dorsal side, males are dark grey with silver-grey patches; sometimes also completely brown or black.

*Weight and size*: Adult males are 200– 210 cm long and weigh 230–270 kg, adult females are 180–190 cm long and weigh 155– 186 kg. Size may vary between populations.

*Lifespan*: Up to 18 years or more (sometimes more than 40 years).

*Diet*: Generalist feeder consuming a wide range of species. Diet may be season- and area-specifc and can consist of sand eels, herring, cod, fatfshes but sometimes also birds, harbour seals and even harbour porpoises.

*Natural predators*: Sharks and orcas (and pups have been observed being eaten by older grey seals as well as by eagles).

*IUCN red list status*: Least concern.

*Special features*: A grey seal pup may gain as much as 2.5 kg per day during the time it receives milk from its mother. This is because the milk contains 50% fat.

### z **Name: Ringed seal (**. **Fig.** 6**)**

### *Scientifc name*: *Pusa hispida*

*Behaviour*: Most ringed seals do not move over large distances, and they regularly return

..      **Fig. 5** Grey seal, *Halichoerus grypus*. © Prof. Krzysztof Skóra Hel Marine Station, University of Gdańsk

to the same haul-out site during the night after their foraging trips.

*Distribution*: Circumpolar distribution throughout the Arctic, and in northern and eastern part of the Baltic Sea.

*Description*: Ring-shaped marks on their fur as adults. Small head with a short cat-like snout with V-shaped nostrils and a plump body. Their fore-fippers have strong, thick claws.

*Weight and size*: Up to 100 kg and 150 cm in length.

*Lifespan*: Up to 19 years.

*Diet*: Prey often consists of schooling species that form dense aggregations. Commonly eaten prey in the Baltic are cod, herring and amphipods. There are regional variations in diet.

*Natural predators*: Polar bears, orcas, foxes, gulls and ravens.

*IUCN red list status*: Least concern.

*Special features*: Ringed seals breed on ice. They build caves or lairs on top of the ice and under the snow by using their strong claws. Here, they give birth and nurse their young. There is a very small land-locked population of ringed seals in lake Saimaa, Finland.

### **3 Adaptation to marine life**

Living in water, marine mammals face challenges such as heat regulation, locomotion, and the need to hold their breath in search for food. As they evolved from terrestrial mammals, they have developed different adaptations to the challenges they meet with their aquatic lifestyle:

Here, we discuss adaptations to these challenges in more detail.

### **3.1 Anatomy**

Most marine mammals are large and roundish. While swimming, their drag is reduced by their torpedo-shaped bodies with short limbs and small or missing external ears. Whales, manatees and sea otters live their entire lives in water. Seals and polar bears live both on land and in water, and their bodies are therefore adapted to both environments.

Whales and dolphins have bodies adapted for moving through water. Any deviations from a torpedo shape are minimized to maintain laminar water fow around the body.  The neck is shortened, hind limbs are suppressed and the forelimbs are fattened. Their body shape minimizes drag by allowing water to fow evenly around them.

Drag consists of several components. The most important ones are pressure and frictional drag. *Pressure drag* results from differences in water pressure between the front and rear part of the body. *Frictional drag* is the force exerted on the surface of the body due to the viscosity of the fuid. The amount of drag depends on the body shape. A sphere moving in the water develops high pressure drag (due to its large cross section area) but low frictional drag (because of the small surfaceto-volume ratio). On the other hand, a long slender body has low pressure drag (due to its small cross section) and a high frictional drag (because of the high surface-to-volume ratio). Moving effciently through water calls for minimizing both types of drag by compromising the body shape between these extremes. The shape with the smallest total drag is an elongated droplet with its largest diameter one-third body length from its front. This is the 'torpedo-like' body shape found in many dolphins and some of the fast baleen whales, such as the blue whale.

The roundish body shape, together with extraordinary thick fur or massive body fat also prevents the animal from becoming cold. Heat loss is an important issue for warmblooded animals in cold water. Newborn marine mammals rapidly gain weight and fat to prevent heat loss. The sooner the newborns can start swimming and hunting on their own, the higher are their chances to survive.

Most marine mammals have a thick layer of fat, known as blubber, below the skin. Blubber has different functions. It acts as an insulator, improving thermoregulation. It affects buoyancy and streamlines the body during hydrodynamic locomotion. Blubber stores lipids, and thus energy, that play an important role during reproduction, parturition and lactation. Many species do not feed during these activities and entirely rely on stored energy to survive. Moreover, lipid is a source of metabolic water that can be used during lactation. Their thick blubber layer causes stranded whales to overheat and dehydrate, which can often lead to death.

The senses need to be modifed when moving from air to underwater. Seeing both on land and in water can be diffcult because air and water have different light propagation properties. The *refractive index* in air is 1, in water it is 1.3. Aquatic mammals display *emmetropia* (sharp vision) while submerged, meaning their eyes are adapted for optimal focusing under water. Both pinnipeds and cetaceans have developed an almost spherical or slightly elliptical lens to provide a suffciently high refractive power under water. Fishes and cephalopods also have spherical eye lenses, so this adaptation for underwater vision has evolved independently many times, so-called *convergent evolution*. In cetaceans, emmetropia in water results in *myopia* (nearsightedness).

In seals, the lens is bifocal to accommodate well both in air and under water. Also, seals have a refective retina that makes their eyes glow at night, similar to nocturnal mammals on land (such as cats and deer). Together with a larger pupil and many rods in the retina, this makes their eyes more light-sensitive than the human eye. Light-sensitive eyes are needed when hunting at great depths and at night. In addition, the highly sensitive vibrissae of seals and the ability for echolocation in dolphins and porpoises supplements and sometimes even substitutes vision, for example, in very murky waters and at night-time.

### **3.2 Whale skeleton**

The curiously shaped skeletons of marine mammals result from evolution of the ones of ancestral mammals that lived on land.

Let us have a look at the skeleton of a harbour porpoise (. Fig. 7). The vertebral column is made up of the cervical, thoracic, lumbar, and caudal vertebrae. Cervical vertebrae are shortened and (in most species of cetaceans) fused. This helps forming the required torpedo shape but limits neck mobil-

..      **Fig. 7** Skeleton of a harbour porpoise, *Phocoena phocoena*

ity. Thoracic vertebrae are restricted to the extremely fexible rib cage, which is remarkable by its very small sternum, resulting in a higher number of 'free-foating' ribs. The thorax is extremely fexible, which allows for lungs being compressed during deep dives.

Lumbar vertebrae extend from the thorax to the anterior part of the caudal vertebrae. Hind limbs and sacrum have been lost during evolution. However, two 'free-foating' sacral bones can be found in some cetaceans.

Caudal vertebrae are easy to identify because they have *chevron bones* (bones of the ventral side of the vertebrae) that insert muscles to move the caudal fuke or hind limbs. At the caudal tip, the fuke is not supported by bones but by connective tissue. Whereas the caudal fn of most fsh is in the vertical plane, the cetacean fuke is in the horizontal plane. This is because cetaceans evolved from terrestrial mammals where fexibility for walking and running occurs in the dorso-ventral plane. Contrary to fshes, the marine mammal dorsal fn is deprived of bones. The dorsal fn works as a keel to keep equilibrium when swimming fast.

The pectoral fns (also called fippers) have evolved from forelimbs originally used to move on land. The limbs of terrestrial mammals are used to support the animal's weight and typically end with claws, nails or hooves. In cetaceans, fippers are used for manoeuvres and locomotion, as paddles. Claws, nails and hooves have disappeared. The fippers are fattened to provide greater strength during locomotion. Moreover, the fipper is shaped as a wing of an airplane and can be used to 'lift' the body upwards towards the surface.

Besides being used for locomotion, fippers, dorsal fn and fuke have a thermoregulation function. Major arteries are located centrally, surrounded by veins. This structural organization is called a *counter-current heat exchanger*: heat from the warm arterial blood fow is transferred to the surrounding veins, heating the blood returning from body extremities. This system can regulate heat in two ways. If the animal needs to cool down, the blood fow to the fn is high. If the animal needs to warm up, it can keep blood closer to the core. Moreover, vascular links between dorsal fn and fukes of dolphins and their reproductive organs allows cool blood to enter the abdominal cavity to regulate the temperature around reproductive organs.

Different modifcations can also be found in the head. An important one is the position of the nares (blowhole). In terrestrial mammals, nares are usually found rostrally (at the front) and directed horizontally or downwards. While swimming, a frontal position of nares is energetically expensive, as you need to lift your head to place your nose out of the water to take a breath. In marine mammals, nares are directed horizontally or even vertically, limiting the effort to breath. The most extreme cases are found in cetaceans, where the nose is directed straight up. Whales therefore do not need to emerge a large part of their head to breathe.

Marine mammal skulls are also characterized by elongated upper and lower jaws that result in a long mouth. Dolphins have numerous sharp conical teeth. Just as for crocodiles, their mouth 'design' is ideal for catching evasive prey, such as fast-moving fsh or squid. In porpoises, the teeth are smaller and spatelshaped. Dolphins and porpoises lack incisors, canines, premolars and molars, typically found in terrestrial mammals (and in seals), and they only have a single kind of tooth.

Baleen whales do not possess teeth, but they have rows of long baleen plates made of *keratin* (a protein also found in nails and hair), growing from their upper gums. Their upper jaw is dorsally curved whereas the lower jaw is laterally curved. Contrary to terrestrial mammals, the cartilaginous joints of the *rostral symphysis* (where the two lower jaws meet) are often missing, allowing the lateral spreading of the lower jaw. In this way they let large amounts of water enter the mouth. The water is sieved through the baleen plates on its way out. The baleen plates are broad and strong at the base but taper into fringes at the tip, forming a large brush that holds smaller prey when water is fushed through them.

### **3.3 Diving physiology**

Most humans can hold their breath for a few minutes and free-dive no deeper than 5–10 m. Marine mammals dive much deeper and for longer durations. For example, Weddell seals can dive deeper than 700 m and stay below the surface for more than an hour. How is this possible?

Diving marine mammals undergo a *dive response*. The heart rate drops tremendously at the start of the dive. In grey seals, it can drop from 120 to 5 beats per minute. Also, blood supply is funnelled to the most vital organs of the animal, such as the brain and heart, whereas most other organs receive little blood and oxygen. The blood volume per kilogram body weight of marine mammals is almost twice as large compared to humans. Both blood and muscles of marine mammals contain larger amounts of oxygen-binding molecules, called haemoglobin (in blood) and myoglobin (in muscles). Whale and seal muscles are deeply red, almost black, because of their high content of myoglobin.

The dive response and other adaptations help marine mammals to store oxygen during their remarkably long and deep dives. Interestingly, humans also have a dive response with a reduced heartbeat, but it is not as dramatic as for marine mammals.  An advantage with emptying the lungs is to prevent gases in the lungs becoming dissolved in blood and tissues at greater depths. If the animal surfaces too quickly, dissolved gases may result in bubbles that can cause blood clots. This problem is called *decompression sickness* and can be reduced if the lungs are emptied before diving. Empty lungs also allow the animal to sink passively without actively swimming upon decent. Many marine mammals can tolerate their lungs collapsing during deep dives through the adaptations of the rib cages explained above.

### **Current topics of marine mammal research**

The dive refex aids in the conservation of oxygen stores in mammals by initiating several specifc physiologic changes during immersion. The dive response was frst described in the 1940s. In these trials, the animal was forced under the surface and its physiological response was recorded. Nowadays, the diving response is investigated in trained as well as wild animals of many different species.

In 2015, Dr Siri Elmegaard and her research team from Aarhus University studied trained harbour porpoises at Fjord&Bælt in Kerteminde, Denmark. Siri tested if a porpoise could voluntarily adjust its diving response depending on the duration of the dive. They designed an experiment where the animal was trained to go for a short or a long dive. The length of the dive was indicated to the animal before the trial using an acoustic signal. During the trials, the animal's heart rate was measured using a device attached by suction cups to the animal (. Fig. 8).

The experiment indicated that harbour porpoises may be able to adjust their heartbeat voluntarily depending on the length of the dive. In this way, the animal can optimize the oxygen demands while diving. If this phenomenon is widely spread in different species, it may have important implications for conservation biology of cetaceans. If the animal is disturbed and forced to dive, for example being scared by a boat, the ability to save energy during the dive may be negatively affected if there is no time for the animal to 'plan' its dive. Therefore, its ability to hunt for prey may be reduced.

### **4 Teaching materials**

### ?**Exercise 1.1: Marine mammals vs. terrestrial ancestors**

Marine mammals are divided into three taxonomic group (orders Cetacea, Sirenia and Carnivora), each with its own terrestrial ancestor. Through evolution, these groups gradually adapted to the marine environment.

Select a marine mammal and compare it with related animals that live on land (. Fig. 1). What is the body shape like? How long are tail and limbs? What do the ears look like? (. Fig. 2)? Where are the nares?

### ?**Exercise 1.2: IUCN red list of threatened species**

Many species of marine mammals are found on the *IUCN Red List of Threatened Species* (7 https://www.iucnredlist.org/). The IUCN Red List is an authoritative guide to the status of biological diversity. The list uses a set of quantitative criteria to evaluate the extinction risk of animals from all regions of the world. There are different threat categories: 'Extinct', 'Critically Endangered', 'Endangered' and 'Vulnerable' consider species threatened by global extinction. 'Near Threatened' concerns species close to being threatened, or species that would be threatened without ongoing conservation measures. Species of 'Least Concern' have a low risk of extinction, and 'Data Defcient' means there is no assessment because of insuffcient data.

Which marine mammals have you encountered, or would you like to encoun-

..      **Fig. 8** Measuring the dive response in a harbour porpoise using a device attached to the back of the animal. © Siri Elmegaard, Aarhus University and Fjord&Bælt

ter? Find out what their status is on the IUCN Red List.

### ?**Exercise 1.3: Mammalian diving refex: How is our heart rate afected by diving?**

Marine mammals must come up to the surface to breathe. When they dive, they need to stay underwater for a long time to enable foraging. Marine mammals rely on anatomical features and physiological responses that have evolved to increase oxygen storage in the body as well as to reduce the use of oxygen for non-essential activities during dives. Marine mammals are also able to funnel their blood fow to essential organs, such as the brain, by constricting some of their blood vessels (*peripheral vasoconstriction*). They also conserve energy and reduce oxygen consumption by lowering their heart rate.

In this exercise, we will see what happens to *your* heart rate when *you* submerge your face in cold water.

Before you do the experiment, think about what you expect will happen. Do you think your heart rate will increase, stay the same or decrease while you are submerged?

### z **Required materials**

### z **Tasks**

You can extend this exercise: Measure heart rates from all students in the class and calculate average change in heart rate. Is there a difference in diving response between males and females, or swimmers and non-swimmers? Induce the diving response by using a bag of ice on different body parts (back of neck, wrists, nose and eyes). What is the effect of such actions on the diving response? Repeat measurements with water of different temperatures. What temperature induce the strongest dive response?

### z **Results**

When mammals dive, their heart rate decreases—this is called the mammalian diving response. Decreased heart rate conserve energy expenditure and oxygen consumption. As a result, mammals can stay underwater longer and therefore also dive deeper. The diving refex is stronger in marine than terrestrial mammals to allow for long dives. Elephant seals have a very strong dive response, and they can stay underwater for more than an hour. They are also helped by a high myoglobin content and a large blood volume (. Table 3).

### ?**Exercise 1.4: Thermoregulation and insulation. How does cold water afect your muscles?**

Heat is lost 20 times faster in water than in air. When marine mammals dive, they need to stay warm and avoid losing too much heat. One of the ways they do this is having a thick layer of insulation called blubber, or a fur covering their body. Cetaceans only have blubber and sea otters only have fur, whereas seals have both. Blubber and fur decrease heat loss. Bowhead whales, for example, have a blubber thickness of up to 50 cm, serving them well in Arctic waters. Sea otters have the densest fur of any mammal.

In this exercise, we will investigate how cold water affects our muscles.

What do you think will happen to your muscle strength when you put your hand in cold water?

Do you think your strength will be affected? Will you get stronger or weaker?

..      **Table 3** (7 Exercise 1.3) Results: Dive response. *bpm* beats per minute. Complete the table and the graph below

### z **Required materials**

### z **Tasks**

1. Measure water temperature of the water.

dynamometer as hard as you can. Record your strength in . Table 4.

If you want to, you can extend this exercise. Collect results from all students and make a graph to see if there is a difference between males and females in the effect of low temperatures. The experiment can also be repeated using different temperatures of water. Is there a difference in muscle strength between submersion in 10 °C compared to 4 °C? Make a graph to explore the relationship between muscle strength and temperature. Instead of a thick cotton glove, use a large plastic glove and fll it with butter to simulate blubber. Use different amounts of butter to compare insulation effciency. To make this exercise less messy and to use less butter, students can put on skin-tight plastic gloves on their hand before putting their hand in the large glove flled with butter. You can then use the same buttered glove for all students.

### z **Results**

Thermoregulation is when an organism can keep its body warm even if its surroundings are colder. Insulation prevents the loss of body heat. When marine mammals dive into cold water, they need to keep their bodies warm. One way they do this is by having blubber and/or fur. This helps insulate the body and allows the animal to stay in the water without getting hypothermia (. Table 4).

### z **Home Pages**

Committee on Taxonomy, Society for Marine Mammalogy; List of marine mammal species and subspecies: 7www.marinemammalscience. org

IUCN Red List: 7 https://www.iucnredlist. org

### **Suggested reading**

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

### **Marine Mammal Acoustics**

*Anja Reckendorf, Lars Seidelin, and Magnus Wahlberg*

### **Contents**

**Supplementary Information** The online version contains supplementary material available at [https://doi.org/10.1007/978-3-031-06836-2\_2].

### n**Learning goals**

### **1 Introduction**

We live in a world of sounds: both natural and human-made (anthropogenic) ones. So do the animals with which we share this planet. Many of them depend on their abilities to navigate the *soundscape* to survive. Marine mammals use sound both for sensing their surroundings and to communicate, sometimes over long distances. Knowledge of bioacoustics and the physics of sound are important, both when trying to understand the lives of marine mammals and also when trying to protect them from human disturbances.

The underwater world contains many different sounds, which can have natural causes like thunder, winds, animal sounds and rain, or be man-made. Marine mammals are well adapted to natural sounds; they react appropriately to sounds of interest to them. 'Noise pollution' is the sum of all additional, unnatural sounds which disturb animals. Sounds are not heard equally well by all animals. An acoustic signal which is important to one species may be without interest or disturbing to another. It is diffcult to assess the impact of noise on animals, as we often need to address the effects on different species separately.

### **2 The physics of sound**

Sounds are waves, somewhat similar to ocean waves you can see rolling in on a beach or that you create yourself in your bathtub. Sound waves can be generated by, for example, a vibrating membrane. Particles of the surrounding medium are set in motion by the membrane and move from side to side, condensing and relaxing the medium. Sound wave propagation is therefore dependent on the oscillation of matter surrounding the sound source. Accordingly, sound needs a medium to propagate. The medium can be a gas (such as air), liquid (such as water) or a solid (such as the seafoor). In outer space, where there is no medium, sound cannot propagate.

The particles in the medium do not actually travel with the sound wave. They oscillate and end up in the same location where they started after the wave has passed. For an animated graphic of sound propagation click 7 here and 7 here.

A sound wave is a *longitudinal* wave. The medium's particles move back and forth parallel to the direction in which the sound wave travels. In contrast, in a *transverse wave* (such as an electromagnetic wave), the particles or feld move perpendicular to the direction in which the wave is travelling. Water waves are more complicated: here, the particles move in a circular fashion.

Check online supplementary animated graphic . Fig. S1.

The easiest described sounds are 'pure tones', or sinusoidal waves. They can also be called signal 'atoms', as any kind of sound can be constructed out of sinusoidal waves. Sinusoidal waves are defned by their amplitude, frequency and phase (. Fig. 1).

**Amplitude**

A*mplitude* measures the local pressure fuctuation created by the movement of the particles, measured in pascals (Pa; or newton/m2, N/m2; . Fig. 1). One pascal equals the pressure generated by 1 newton over an area of 1 m2. In air, one pascal is a very high sound level, similar to noise generated by cars on a highway at a close distance. Compared to the atmospheric pressure, which is around 100 kPa at the surface of the Earth, the pressure fuctuations generated by most sounds are small.

..      **Fig. 1** Left top: A sound wave, depicted as a function of time (recorded at a certain position). Left bottom: The same sound wave, depicted as a function of range (recorded at several locations). Right top and bot-

### **Frequency**

*Frequency* is defned by the number of wave cycles in 1 s, measured in hertz (Hz). Thus, 1 Hz is one cycle/sec, while a kilohertz (kHz) is one thousand cycles per second.

The frequency is closely related to the wave's *pitch*, which is the animal's perception of how 'high' (*treble*) or 'low' (*bass*) the sound is. Decreasing the frequency lowers the sound's pitch and makes it more 'basslike', while increasing the frequency raises it (making the sound more 'treble-like'; . Fig. 1). Different species of animals have their best hearing at different frequencies: humans hear best at 2–4 kHz, while porpoises hear best at 100–140 kHz.

### **Intensity**

Another useful unit is the *intensity* of the wave, which is defned as *I* = *p*2/(*ρc*), where *p* is the sound pressure (measured in Pa), *ρ* is the density of the medium (in kg/m3) and *c* is the speed of sound in the specifc medium (in m/s). Intensity is measured in units of W/m2 (watts/metre2). Intensity is related, but not identical to, how loud humans and other animals perceive sound (for humans, the latter is called the *loudness* of the sound).

tom: a sound wave of higher frequency, depicted as a function of time and range. A sinusoidal sound wave is characterized by its 'height' (amplitude), frequency and phase (determining at which time the peak occurs)

It is important to understand the basics of pressure and intensity before we move on to discuss how well animals hear. When comparing hearing in air and underwater, it makes a difference if we report hearing thresholds (i.e. the weakest sound an animal can here) in pressure or intensity units. As we will see below, when measured in units of pascals, underwater hearing thresholds seem higher than aerial ones. However, this does not necessarily mean that marine mammals hear worse underwater than terrestrial mammals hear in air. The difference is to a large part a matter of the composition of the medium in which the sound propagates. Because water is denser than air, a pascal of sound underwater carries less intensity than a pascal of sound in air. Thus, comparing in-air and underwater sound levels is by no means trivial. When reporting thresholds in units of intensity, some marine mammals (such as seals, which live a semi-aquatic lifestyle) have similar hearing thresholds in air and underwater, while others (such as dolphins, which are entirely aquatic) have poorer hearing in air but extremely sensitive hearing underwater.

### **Sound levels expressed as decibels**

Because the span in sound pressures that can be observed in nature is huge, the sound amplitude scale is often compressed into a logarithmic scale, known as *the decibel scale*. Decibels are calculated as

### dB = 20 ( ), <sup>10</sup> <sup>0</sup> log / *p p*

where log10 is the base-10 logarithm, p is the measured pressure (in Pa), and *p*<sup>0</sup> is the so-called *reference pressure*, which is *p*0 = 20 μPa (micropascals) for airborne sounds, and *p*<sup>0</sup> = 1 μPa for underwater sound. The reference pressure in air is close to the human hearing threshold at 1 kHz. In this way, the hearing threshold of humans at this frequency is 0 dB re 20 μPa (try to calculate this yourself). The underwater reference pressure does not have any biological meaning but has been hand-picked by a scientifc committee. To indicate which reference pressure is used, in-air sound pressure levels are reported as dB re 20 μPa and underwater as dB re 1 μPa (. Fig. 2).

Because of the differences in reference units as well as in speed of sound and density of air and water, it is not easy to compare in-air decibel levels with underwater ones. A sound signal of equal pressure in both media is 26 dB higher in water than in air (due to the different reference pressures), and a sound of equal intensity is 62 dB higher in water than in air (due to different reference pressures and impedances).

Let's compare the decibel scales in air and underwater (. Fig. 2). In air, the scale of audible sounds spans from the hearing threshold, which for most mammals is lower than 20 μPa up to about 100 Pa, around the *threshold of pain*. This means a difference of 6–7 orders of magnitude between the lowest and highest point of the audible scale. In decibel units, the biologically relevant in-air scale is from a bit less than 0 dB to more than 130 dB re 20 μPa. In water, the lowest hearing thresholds of marine mammals are about 100–1000 μPa (which in decibels become 40–60 dB re 1 μPa), while the threshold of pain is probably beyond 1 or 10 kPa (180–200 dB re 1 μPa). The loudest sounds that can be produced in air, such as sounds from rife shots and jet engines, can be up to around 180 dB re 20 μPa. Beyond this sound level, the air molecules cannot maintain the extremely large particle motions and start to behave in ways not easily described by physics. Underwater, the loudest animal sounds are produced by sperm whales, which sometimes click at about 240 dB re 1 μPa, if recorded at a distance of 1 m. Even louder sounds exist, but these are mainly of human origin, such as the sounds from sonars and underwater explosions. Remember though that to directly compare the sound intensity in air and underwater, you have to subtract 62 dB from the underwater decibels.

Another important feature of a sound wave is its frequency content. Young humans can hear from about 20 Hz up to 20 kHz. With age, the upper hearing limit is rapidly decreasing so most people older than 50 years have a hard time hearing sounds above 10 kHz. Humans hear best at 2–4 kHz, which is signifcantly higher than the frequencies where we emphasise our speech, at a few 100s of hertz (. Fig. 3).

#### ► **Example**

Human hearing is not fne-tuned to the frequencies at which we communicate. But why do we speak at different frequencies than our best hearing occurs? A crowded bus ride may give you a hint. You can easily continue talking to your friend nearby, in spite of all the noise of other people talking around you. If a baby starts crying however—and babies cry in the 2–4 kHz range where humans hear best—you may stop your conversation to see if any help is needed or soon consider switching to another bus to continue your conversation. Our hearing seems to be evolutionary tuned to listen in for infants that need to be comforted. ◄

The speed at which a sound wave travels is different for every medium. It depends on the density, temperature and a few other parameters, like ambient pressure and (for underwater

..      **Fig. 2** In-air and underwater decibel scales. The reference pressure for airborne sounds is dB re 20 μPa, while dB re 1 μPa is the reference pressure for underwa-

sound) salinity. In air, the speed of sound is 330–340 m/s, and in water 1450–1500 m/s. Thus, sound travels about 4.4 times faster underwater than in air.

The different acoustic properties of air and water have implications for the way we perceive sound in the two media. For example, an important cue when listening for sounds is to determine their direction. Many animals,

ter sounds. The water dB scale is shifted by 62 dB, so that in-air and underwater sounds at the same vertical location have the same acoustic intensity

and humans, do this primarily by gauging the difference in timing or intensity of the sound received at each ear. Both timing and intensity differences will become smaller in water. Due to the high speed of sound transmission underwater, sound is received at both ears closer in time than for airborne sounds. Furthermore, for a signal with a certain frequency content, the wavelength is longer

..      **Fig. 3** Left: the human hearing threshold in blue, and threshold of pain shown in red. On the right, the audiogram of a harbour porpoise (green) is compared to a human audiogram (blue). The audiogram thresh-

underwater than in air, as the wavelength is the ratio between speed of sound and frequency. Due to the longer wavelengths, and the similarity of fesh, bone and water density, the animal's head is less effcient in shielding signals underwater than in air. Therefore, intensity difference cues between the two ears to determine the direction of a sound source become less effcient in water. Terrestrially adapted animals, and humans, have a very hard time determining the direction of a sound source underwater. Marine mammals, however, have certain anatomical and physiological adaptations to tease out the direction of underwater sounds. Porpoises, for example, have acoustically decoupled inner ears: they are detached from the skull and surrounded by air compartments.

### **3 Hearing and sound production**

Marine mammals use sound to communicate over short and long distances, to acoustically sense their surroundings and navigate underwater, locate prey and mating partners, and to avoid predators.

Depending on the species, the upper hearing limit differs tremendously. Seals can hear well up to about 20 kHz in air, whereas their underwater frequency range is more than doubled, up to 50 kHz. Their hearing is adapted to life both in air and water. Toothed whales hear well underwater but poorly in air.

olds have been shifted so that the intensity level is the same for the underwater and in-air hearing thresholds. Left fgure from Larsen and Wahlberg (2017), right fgure from Wahlberg et al. (2015)

Many of them can hear even higher frequencies than seals. The record holder are small odontocetes such as the harbour porpoise, which can hear frequencies higher than 140 kHz. Porpoises hear best in the frequency range where they produce their echolocation signals (. Fig. 4). Their hearing sensitivity at these frequencies is among the best sensitivities found in any aquatic mammal.

Marine mammals produce a large repertoire of different sounds. In most cases you can quite easily detect the difference between sounds from a seal, a dolphin and a baleen whale. Seals produce sounds both in air and underwater in the same frequency range as humans, mainly for communication and during mating activities. Dolphins produce clicks, burst-pulsed sounds and whistles. Some of these signals are produced in the frequency range audible to humans, but they often also contain higher, so-called *ultrasonic*, frequencies. Baleen whales produce extremely lowfrequency sounds. Blue and fn whales regularly sing below 20 Hz, which is the lower frequency limit of human hearing; sounds below this limit are called *infrasound*.

Sound is one of the most useful ways of communication, above and especially underwater. Many marine animals have good eyesight, but even in the clearest tropical waters, visibility is restricted to a few tens of metres. Sound on the other hand travels much further. Many low-frequency sounds may be often heard over very large distances. Many

..      **Fig. 4** Hearing (in black) and sound production (in grey) ranges in a selection of animals

marine organisms have developed ways to produce and detect sound. Whales, for instance, have developed distinct communication strategies and orientation systems, which allow them to fnd prey and communicate with conspecifcs over great distances. For example, blue whale calls have been heard at ranges of tens or even hundreds of kilometres.

Toothed whales (odontocetes) use echolocation. They produce very high-frequency sound waves by forcing air past so-called *phonic lips* into the air sacs of their nasal passages. The clicks are transmitted through sound conducting acoustic fat of different densities in the front of their head (the *melon*), directing the sound in to a narrow beam. The emitted sound waves travel through the water until they reach an object, such as a prey item or an underwater obstacle. The object refects some of the sound energy, creating an echo. The returning echoes are mainly received through the whale's lower jaw, containing fat which channels the perceived sound towards their inner ear complex (Fig. 5). Thus, by emitting ultrasonic sound pulses, odontocetes listen for echoes and can detect objects underwater. They can basically 'see' their environment through the sound waves they produce, in some ways similar to the echo sounder on board a ship.

..      **Fig. 5** Sound production and reception in a harbour porpoise. The purple structure in the forehead is the acoustic fat of the melon, which channels the emitted clicks (green). The sound is refected by the fsh (orange

Baleen whales and seals do not use echolocation, but they still rely on sound and hearing for survival. Seals, and probably also baleen whales, produce sounds with their larynx and some additional air sacs, similar to terrestrial mammals. Besides good underwater and in-air hearing abilities, pinnipeds (true seals, walrus and eared seals) can perceive water disturbances with their whiskers to detect water movements generated by swimming fsh.

### ► **Example**

Marine mammals have evolved adaptations that allow them to use sounds both to detect prey and to avoid predators. An interesting example is the frequency range of sounds and hearing of harbour porpoises compared to one of their predators, orcas. Orca sounds consist of a wide range of whistles and pulsed calls, with fundamental frequencies as low as a few kHz and harmonics beyond 100 kHz. Their best hearing is around 40 kHz. Harbour porpoises hear best between 120 and 140 kHz but can also hear sounds as low as a few kHz. Harbour porpoise signals, on the

dashed line) and mainly received through the lower jaw. The red shading indicates the mandibular acoustic fat close to the ear bone. © Annika Toth

other hand, do not contain much energy at frequencies below 100 kHz and are centred around 120–140 kHz, where they hear best. It is believed that porpoises have adapted their sound production to higher frequencies, which are out of the hearing range of orcas. This prevents orcas from hearing sounds from their potential prey. Porpoises, on the other hand, can hear the sounds from nearby orcas and may therefore be able to avoid being captured. However, orcas have highly specialised hunting strategies and often stop making sounds before starting a hunt, making it more diffcult for their prey to evade the attack.

Some of the strongest reactions to sounds played to marine mammals occur when a playback signal sounds like a natural predator. Again, orcas are an interesting example. There are different types of orcas: some feed on marine mammals, while others only eat fsh. The different orca types also have different sound repertoires. Orca sounds were played back to Canadian harbour seals, which are regularly preyed upon by orcas. The seals responded aversively to sounds from mammal-eating orcas, but they did not respond to calls of fsh-eating orcas. ◄

There are many ways humans utilise the oceans, and many of these activities introduce sound into the natural environment. Underwater noise pollution is caused by, for example, recreational boats, commercial shipping, windfarms, oil rigs, underwater exploration for oil and gas, and military activities. Highly utilised areas often have particularly high levels of underwater sound. Noise pollution may however be just as bad or even worse in quieter areas due to a smaller degree of *habituation* (become accustomed to a behaviour or condition) in animals living in such environments.

Because many marine mammals use acoustics as an important way to detect prey, to communicate or to orient themselves, a functional hearing system is of uttermost importance to them. To a certain degree, marine animals can increase the sound level or frequency range of their communication signals to make them more audible under noisy conditions. If communication sounds are masked by anthropogenic noise, they may not be able to hear calls of conspecifcs to fnd mating opportunities or food. This can potentially play a major role in individual or species survival.

Noise can also disturb the animals' natural behaviour, make them leave their known habitat to fnd more silent locations, interrupt feeding, change diving behaviour, maybe even cause reproductive impairment through acute and chronic stress, and lead to temporal or permanent changes in distribution. Loud continuous or impulsive noise can damage hearing at certain frequencies, temporarily or permanently, which can result in failure to hear important signals, such as an approaching ship or a certain frequency component of mating calls. Additionally, certain types of anthropogenic noise, like military sonar, can lead to death when deep-diving whales attempt to escape or evade the sound by ascending too quickly.

Hearing impairment is caused by exposure to loud sounds, also to less intense but continuous noise over longer periods. If temporary hearing impairment occurs, it is called temporary threshold shift (TTS). A TTS is a passing increase in the auditory threshold resulting in momentary hearing impairment. We humans know this phenomenon as the 'discotheque effect', as our hearing is temporarily reduced after exposure to loud music. A permanent hearing threshold shift can occur as a result of repeated TTS events or from a single exposure to an intense sound. Both humans and marine mammals can become permanently less sensitive or even deaf to sounds of certain frequencies after loud or long exposure of sound.

### ► **Example**

Another problem that can be investigated with bioacoustics is collisions between ships and whales. The *International Whaling Commission* runs a global ship strike database (7 http://iwc. int/ship-strikes), and it shows an increasing number of records over the last years. Why do whales and manatees get hit by vessels instead of avoiding them?

One of the reasons may be that the main noise source of a boat is the engine, which is situated in the stern of the boat. The hull is shielding the engine noise towards the front, so straight ahead of the boat the noise level is substantially reduced. If the animal approaches the boat sideways, the noise level drastically increases, making it aware of the approaching danger. If an animal forages or sleeps straight ahead in the ship's pathway, it may not perceive the sound of the engine and propellers, making it more vulnerable to collide with the ship, while whales startled by noise tend to surface rapidly, putting themselves in greater danger for collisions at the surface. The animal may even seek out a location straight ahead of the boat, to avoid too much noise from the approaching vessel. Additionally, underestimation of vessel speed, and maybe also hearing impairment, could also explain why animals are increasingly colliding with boats. Also, the number of fast-moving boats in waters frequented by cetaceans and manatees are constantly increasing. ◄

Continuous stress through noise pollution can supress the immune system and make marine life more vulnerable to infectious diseases and parasitic infections. Many marine mammals hear and communicate at frequencies different from the ones used by humans. They may therefore be affected by sounds that we cannot hear. Thus, when evaluating noise effects on marine mammals, it is important to use broadband recordings and analysis systems that can detect sounds also outside the hearing range of humans.

We need more research on the current levels of noise in the oceans, and how noise affect marine fauna. Politicians and decision makers need to take the results from such research into account when judging if mitigation measures are needed. The environmental impact assessment of new marine activities should include potential effects of noise emission. When needed, successfully tested sound mitigation measures should be implemented. Such measures could include effective animal warning devices used prior to military sonar, or so-called 'bubble curtains' used during construction work (see Exercise 5). One could also envision the development of quieter ship engines, changing shipping routes and harbour entry lanes, or speed reduction zones.

### **Tip**

Buying more local goods instead of having everything shipped from abroad is another way to improve the current situation of extensive shipping (if you want to learn more about the marine traffc density in your area, you can search on the internet for links displaying marine traffc in your country's coastal zones or exclusive economic zone). Thus, by adjusting our personal behaviour we may improve the situation not only for marine mammals but also for the marine environment in general.

**Current topics of acoustic and noise pollution research on marine mammals**

How can we use acoustic research to protect marine mammals and to learn more about them?

We still do not know much about how marine mammals communicate and orientate themselves underwater with sound. Researchers work with trained animals, sometimes blindfolded to ascertain they only make use of auditory cues during the experiments. In this way, it can be shown that, for example, blindfolded toothed whales are able to fnd their way through a maze or detect and catch prey. This is considered the classical evidence of their ability to echolocate.

We can also investigate the hearing abilities of trained animals by hearing tests. This can be done in the same way as with humans: When a sound is played during an experiment, the animal is trained to indicate if it heard the sound by putting its snout on a certain *response symbol*. The hearing abilities of stranded individuals can be checked through measurements of their *auditory brainstem response* (ABR), small electrical potentials originating from the brain in response to auditory stimuli. The electrical potentials are recorded from the skin of the animal with electrodes embedded in suction cups, which are attached to the head. In this way we can measure if animals can hear a certain sound without being trained for a hearing test. The same methodology is used to detect hearing problems in newborn human babies. In stranded marine mammals, ABR measurements are sometimes used as a health parameter: The animal needs to be able to hear the species-specifc frequencies to be released back into the wild.

Another approach to study the impact of noise pollution on marine mammals is using acoustic data loggers that can be deployed in critical habitats or along shipping channels. The data loggers record calls of the target species and ambient noise. In this way, scientists can better understand species distribution and habitat use without having to be continuously present on a boat for observations. Real-time passive acoustic sensors can transmit the sounds of seals and whales via satellite or radio links to a unit, which informs captains that animals are present, and that caution has to be practiced once an alert has been sent.

If you want to learn more, *Discovery of Sound in the Sea* offers great further learning materials and updated information on their web page 7 https://dosits.org/ (only in English).

### **5 Teaching materials**

### ?**Exercise 2.1: The frequencer**

Have you ever wondered why some people hear sounds that others cannot hear? What are the frequency limits of your hearing, and are both of your ears equally sensitive to sound?

By listening to sounds of different frequency and intensity, you can register the frequency limits of your hearing. You can then compare your hearing limits with the ones of other animals (such as marine mammals; see an example in . Fig. 3) to better understand in what ways our hearing differs from theirs.

### z **Required materials**

### z **Tasks**

### z **Results**

The result will be your own, personal frequency limits of your hearing.

Compare your results to other students and repeat the experiment with people of different age classes, like your parents and grandparents. Can you see differences in the hearing limits?

The results will not be as accurate as an audiogram measured by professional audiologists, as the played back sound level will depend on the type of loudspeaker.

### ?** How could you orientate yourself? Do you think you could teach yourself *human echolocation*?

Not only bats and toothed whales use echolocation—humans can also learn to echolocate, and some can become quite good at it. Visually impaired humans sometimes spontaneously learn to echolocate to compensate for their lack of vision. This phenomenon was formerly called 'facial vision' but is now known to be echolocation. Interestingly, one of the best human echolocators known, Daniel Kish from California, remembers learning this skill while being only a few years old. He only understood that he was using a method comparable to bats and dolphins when 20 years old. If you want to know more about this technique, check out 7 https://visioneers.org/.

### z **Tasks**

### z **Results**

When approaching a wall, the blindfolded student will notice a change in the returning echo of the emitted click sound. This should cause the student to stop or to turn, instead of walking into the wall. When stopped in front of a wall, the student should slowly reach out for them to appreciate the actual distance. You can also leave an opening in the shielded circle, that has to be found by echolocation. Afterwards, the students can disperse in the room and the blindfolded one can try to locate them by echolocation and fnd a way to navigate around them. Additionally, if the students are trained to echolocate, different materials can be tested, since harder surfaces (e.g. a white board) refect sound in a different way than softer ones (e.g. cardboard) do.

### ?**Exercise 2.3: Name the sound**

How much do you know about animal, environmental and anthropogenic sounds? And how much do you know about underwater sounds? Do you think you can discern natural from man-made underwater sounds

..      **Fig. 6** Human echolocation (7 Exercise 2.2). The students stand in a circle, shielding themselves with panel 'walls', while a blindfolded student is standing

inside the circle. The blindfolded emits click sounds while slowly walking around, trying to fgure out where the 'walls' are by listening for returning echoes

easily? You have surely listened to songbirds, and you may have tried to identify them by their song. There are many more natural sounds to explore. In this exercise, we focus on some underwater sound examples that are completely unknown to most people. Can you guess the origin of those sounds?

### z **Required materials**

### z **Tasks**

1: Listen to the automatically played sound and repeat it by clicking on the small speaker symbol.

Guess its origin before continuing to reveal the four provided possible answers.

Was your frst guess represented in the four answer options?

If not, choose A, B, C or D as your answer. If so, was your answer correct?

### z **Results**

You will see how easy or diffcult it is to discern natural and man-made sounds from each other. Probably animals often have the same diffculties as we do.

### ?**Exercise 2.4: Build your own hydrophone**

Do you think that there is a simple technological way for us to hear sounds inside natural bodies of water? If so, what do you think you will hear down there?

Hydrophones are underwater microphones that can be easily built from a few components. With a hydrophone, you can listen for animals, such as whales and seals communicating with one another, or just the general underwater soundscape, sounding so eerie and unfamiliar to most of us 'land dwellers'. And with the right materials, it only takes 30 min to build your own hydrophone.

### z **Required materials (**. **Fig.** 7a**)**

5 This exercise includes soldering, so proper safety precautions need to be in place and care is advised. A fume hood or other ventilation system is needed, students have to be extra cautious while the soldering iron is connected and accordingly need supervision for the soldering part of the task. If soldering is considered too dangerous or unfeasible during a class exercise, this part can also be prepared prior to the experiment.

### z **Tasks**

..      **Fig. 7** Build your own hydrophone (7 Exercise 2.4). **a** All necessary materials laid out on the working desk. **b**

sound-sensitive crystal. Always take caution not to inhale solder fumes (. Fig. 7b).

3. Attach the amplifer to the connector, connect the output of the amplifer to the loudspeaker or headphones and switch it on. Now perform a 'tap test': tap gently on the crystal—you should hear a sharp noise through the loudspeaker.

Soldering the cable, one strand on each side of the piezoelectric crystal. **c** After placing the crystal in the oil flled container, everything is connected and tested

### z **Results**

The vegetable oil does not conduct electricity, but it conducts sound waves: therefore, the underwater sound waves can reach the piezoelectric crystal. The crystal transforms the pressure into voltage. The voltage is picked up by the cable strands and gets amplifed before being transmitted to the loudspeaker. Thus, sound can be transmitted to the speaker without short-circuiting the crystal.

#### **Tip**

Bring the hydrophone out to a dock by a lake or by the sea and sink it into the water. Take caution to keep the rest of the electronics dry. Now, listen to the sounds. You can also splash the water surface with your fngers or throw a small stone into the water. You can hear if you can detect fsh or other natural sounds, or perhaps you can hear a passing boat?

### ?**Exercise 2.5: Build a bubble curtain**

Imagine you want to build an offshore wind farm, to generate renewable, clean energy. How would that effect aquatic life in the area? What can be done to protect them from harmful noise impacts?

Bubble curtains are noise mitigation measures that are used to dampen and absorb sounds of underwater constructions to prevent extensive hearing loss and noise harassment of marine mammals close to the construction location. Sometimes, huge bubble curtain systems are built around construction sites and activated when necessary. If you are interested in more information and pictures regarding this topic, search the internet for, for example, *The Big Bubble Curtain* by BBC.

In this experiment, we will build a miniature bubble curtain to show the effectiveness of such a mitigation device.

### z **Required materials**

5 Air compressor (it can be a very simple, cheap aquarium pump, but the stronger the compressor, the better the result).

### z **Tasks**

The students should be silent during the entire experiment.

..      **Fig. 8** Build a bubble curtain (7 Exercise 2.5). **a** Air bubble tube for aquaria, the air holes are clearly visible. The end needs to be sealed; here we use glue. **b** Complete bubble curtain setup with aquarium pump, connective

hose and air bubble tube. **c** Running bubble curtain experiment. The red alarm functions as free-foating underwater sound source. The white, self-built hydrophone is used to measure sound levels inside and outside the curtain for comparison

6. Think about the implementation of a bubble curtain on a large scale. Which materials are needed to make it work? Are there other problems that could be caused by using a bubble curtain?

### z **Results**

When the bubble curtain appears, there should be a clearly detectable drop in the sound level. Due to the limited amount of air holes in the commercially available tubes or depending on the strength of the used compressor, the effectiveness of this experiment is often not huge, but the students can get a good feeling for how effective it can be if more air bubbles and dual or triple layers are used.

For Task 6, additional ship traffc around the construction site, electricity supply to the construction site and running of loud generators should be considered, as well as huge hoses on the sea foor, likely damaging plants and microhabitats. Still, the impact of piledriving is usually considered more dangerous and needs mitigation measures, even if they come at additional environmental costs.

### **Tip**

By submerging the active sound device in the water, you can try to explain a hard sound barrier and sound reflection. The density difference between the two media (air and water) is several hundredfold; therefore, the two media form a *boundary* and we cannot hear what is happening underwater while we listen in air. This is why bodies of water usually appear silent to us. In this case, we still hear the sound of the panic alarm, because it is loud and the container walls transmit the sound back into the air. To prove this, let the alarm sink to the bottom, and the transmission of the sound will appear louder. Try the bubble curtain again with the sound device lying on the bottom. It will not work well, due to sound being transmitted by the container walls. You can also use a stethoscope to listen to the sound from inside the aquarium and better avoid the effects of the walls.

If the device would be submerged in the ocean where the sound is not redirected and transmitted, we would not hear it at the water surface. If we are submerged, we can hear more of the sounds present underwater. However, the airwater sound barrier still exists inside our ears, so we do not hear underwater sounds as good as in-air sounds. Many underwater sounds are outside the human hearing range but within the hearing range of marine life. Often, we may not think enough about the potential damage caused by underwater noise because we do not hear the noise that well.

### z **Home Page**

All animated graphics by Dan Russell are available under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. 7 https://www.acs. psu.edu/drussell/

### **Suggested reading**

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# **Whaling, Seal Hunting and the Efect of Fisheries on Marine Mammals**

*Miko*ł*aj Koss, Martin Stjernstedt, Iwona Pawliczka, Anja Reckendorf, and Ursula Siebert*

### **Contents**

**Suggested reading – 47**

**Supplementary Information** The online version contains supplementary material available at [https://doi.org/10.1007/978-3-031-06836-2\_3].

### n**Learning goals**

### **1 Introduction**

Marine mammals are important top predators in the world's oceans. Many species are opportunistic feeders, catching the most abundant, convenient and easiest caught prey, depending on region and season (. Table 1). For example, Baltic grey seals (*Halichoerus grypus*) feed mostly on herring, sprat and cod in the Baltic Proper, but further north in the Bothnian Bay they also feed on whitefsh and salmon.

Apart from being an important part of marine food webs, marine mammals are indicators for the state of the marine environment. This is one of the reasons why stranded marine mammals undergo extensive postmortem investigations, including stomach content analysis and including monitoring levels of toxic substances in various tissues. Large abundances of whales or seals in certain regions indicate good environmental conditions. For example, decreasing numbers of hauled-out seals could indicate that various

Sources: Bergman (2007) and Rojano-Donate et al. (2018). The food consumption is affected by body weight, diet composition and season

human activities and environment alterations prevent them from thriving. Additionally, if marine mammals maintain a low *fertility rate* (having very few or no offspring) or a high level of diseases in a particular region, it is likely that their environment and prey are contaminated with toxic substances. Common toxic substances are persistent organic pollutants such as pesticides (for example DDT) or industrial chemicals (such as PCB). Similarly, the presence and good health status of marine mammals indicate a marine environment of high quality.

Overlap in human and marine mammal diets can lead to competition and result in bycatch, catch depredation and fshing gear damage. To study such issues, it is essential to know the diet of marine mammals in the area of interest. The identifcation and size of fsh prey can be derived by measuring *otoliths* (fsh ear stones; see 7 Exercise 4.1 and . Fig. 4) often found in the stomach or faeces of marine mammals.

### **2 Bycatch**

Fisheries operate in many areas that are natural foraging grounds of marine mammals. This inevitably leads to interactions and conficts between fsheries and marine mammals. One type of interaction is incidental entanglement and death of marine mammals in fshing gear, called bycatch. Another major issue is gear and catch damage caused by marine mammals.

In the Baltic and North Sea region, bycatch is one of the major anthropogenic threats to marine mammals, although quantitative estimates of mortality in fshing gear are scarce. A few extensive reports suggest that the bycatch rate can be higher than reported in offcial statistics. For Baltic grey seals, the yearly bycatch may be higher than 2300 individuals. For some species, there has luckily been a documented reduction in bycatch: In the 1990s, the estimated bycatch was 7,000 harbour porpoises (*Phocoena phocoena*) in the Danish part of the North Sea. Nowadays, the estimated porpoise bycatch is lower due to altered fshing efforts as well as the use of *acoustic deterrent devices* (so-called *pingers*). Still, bycatch remains one of the largest threats to marine life, including marine mammals, in many regions, including the North Sea and the Baltic.

### **Bycatch**

Incidental catch of non-target marine species in fshing gear. Marine mammals, fshes (non-targeted species and undersized specimens), birds and turtles, as well as invertebrates constitute a substantial part of bycatch in certain fsheries.

The main fshing gear responsible for marine mammal bycatch are gillnets, which are anchored on the sea bottom. Gill nets form very long, nearly transparent walls in the water column, constituting traps for marine mammals and other animals. Unfortunately, incidents of marine mammal bycatch, although fairly numerous in certain regions, are rarely reported by fshermen and not always adequately monitored, which makes a proper assessment of this threat diffcult. Frequently, stranded cetaceans show characteristic net marks around their heads or other parts of the body, indicating previous entanglement in fshing nets (. Fig. 1). Postmortem examinations often reveal a full stomach and bleeding in several organs, most likely caused by death struggle. Net wounds around the neck, snout or fippers are also observed in seals. Even though scientists sometimes judge bycatch as a likely cause of death, they are often unable to obtain information on the location, date and type of fshing gear in which the animal may have been caught. Such data are crucial for the implementation of effective conservation measures for the species in question.

Changing fshing gear to newer types known to be less harmful to marine mammals does not always provide an environmentally responsible or economically feasible solution. For instance, bottom trawling (dragging a trawl over the seafoor) has little direct impact on marine mammals but causes severe damage to *benthic* communities, as well as it may result in a large bycatch of juvenile fsh and invertebrates. Thus, switching to 'marine mammal friendly gear' will not always solve the overall problems.

The worldwide demand for fsh will increase in the coming decades. Due to overfshing and the huge quantity of bycaught and discarded smaller sized fsh, the amounts of fsh obtained by fshing no longer meet the global need. Therefore, most of the fsh consumed today is farm-raised, not wild. Aquaculture is regarded as a solution to the burning issue of ocean overfshing. The main advantage of aquaculture is that, unlike fshing, it relies on bred and harvested fsh, not on depleting wild fsh stocks. Prospects of aquaculture are promising, but some types, such as offshore open systems, pose serious environmental threats, resulting in unhindered interactions between farmed fsh enclosed in cages or netting systems and the surrounding environment. Aquaculture can spread diseases, parasites and chemicals (e.g. antibiotics) into

..      **Fig. 1** Left: Bycaught harbour porpoise with net marks around the snout. © Katarzyna Jęczkowska. Right: Stranded grey seal entangled in fshing net. © Mateusz Puzdrowski

the wild. Faeces and nutrients released into the environment may cause the rise of algal blooms and eutrophication. Damaged cages result in farmed fsh escaping. Escapees of farmed fsh not belonging in a certain habitat can compete for food and place with indigenous species. The escaped fsh can also interbreed with the wild stocks, which may lead to questionable mixing of gene pools. In addition, aqua-cultured fsh needs to eat. To grow them to harvestable size, a large amount of additional smaller fsh is needed, which is usually wild-caught. Thus, there are both positive and negative sides of marine fsh aquaculture.

### **3 Seal-safe fshing gear and catch damage**

Marine mammals can cause catch loss or gear damage by feeding on fsh caught in nets (. Fig. 2). Since the 1990s, the confict has escalated between Baltic fsheries and grey seals. The grey seal population of historically about 100,000 individuals suffered a drastic decline in the 1970s after decades of hunting and exposure to pollution. After being protected in the 1980s, the Baltic grey seal population have recovered, reaching over 40,000 individuals in the late 2010s. Meanwhile, the biomass of several commercial fsh species has decreased in the Baltic for various reasons, and seals have increasingly been perceived as being in competition with fsheries. The seals may affect the fsh stocks not only by directly feeding on them but also through the spreading of parasites, for which seals and fsh are part of the life cycle.

The most severe gear damage and catch loss due to Baltic grey seals is reported from the Gulf of Bothnia. The problems are worst in coastal fsheries using static fshing gear, such as salmon traps, as well as in gillnet fsheries for herring and whitefsh. Fisheries using active fshing gear, such as trawling, are less affected. Various methods have been tested to minimise the seal-fshery interactions. One option is fnancial compensation for seal-induced catch damages. However, this solution is short-term and does not solve the actual problem. Additionally, it does not take the seal mortality in fshing gear into account, unless special constructions preventing seals from becoming entrapped in the gear are used voluntarily by fshers. The implementation of so-called *alternative fshing gear* may both reduce catch and gear damage, as well as bycatch of seals.

### **4 Bycatch mitigation measures**

There are four main approaches to reduce bycatch of marine mammals:

..      **Fig. 2** Marine mammals, in particular pinnipeds, can cause severe catch loss or gear damage by feeding on fsh entrapped in fshing nets. © Annika Toth

The most effective way to curb bycatch is to reduce fsheries that incidentally catch marine mammals. Implementation of he total allowed fshing effort using, for example, gillnets, is nowadays made.

Acoustic deterrent devices of the so-called *pinger* type (. Fig. 3) are effective in minimising bycatch of harbour porpoises. Pingers emit acoustic signals of rather low intensity, with frequencies between 10 and 180 kHz. They are attached to gill nets at a few hundred metres distance from each other. According to EU legislations, fshing vessels above 12 m length using some types of gillnets in certain areas are obliged to use pingers. Unfortunately, gillnets are widely used in Baltic waters, and the areas and gill net types where pingers are mandatory does not always overlap with the areas having the highest abundance of harbour porpoises.

Pingers can almost entirely eliminate bycatch of harbour porpoises, when used properly. However, it is unclear if porpoises habituate to pingers in the long run. Also, pingers may not scare off but instead attract grey seals to the nets. This complicates the effciency of pingers in regions where seals are abundant, since the pingers may act as *dinner bells* for seals and thereby intensify seal depredation. Since seal depredation is already a large problem in the Baltic Sea, Baltic fshermen are reluctant to use pingers. Currently, there are attempts to develop pingers that are inaudible to seals but audible to porpoises.

Temporal and spatial closure of fsheries is another way to successfully reduce bycatch. The deployment of gillnets could be banned during certain times of the year and in locations important for marine mammals. Proper implementation of such regulations requires thorough information on the distribution of marine mammals and an in-depth understanding of the needs of fsheries in the designated area.

Another way to reduce seal-induced catch loss is the development of new fshing techniques. There are different types of alternative fshing gear that have been introduced in the Baltic Sea, for example, *pontoon traps* and *cod pots*. The construction of alternative fshing gear aims to protect the fsh catch from the seals, but also the seals from being bycaught. In addition to new fshing devices and techniques, traditional fshing gear could be modifed by using stronger net materials, and wire partitions or grids in the entrance of traps and fyke nets.

Fish and seafood consumers need to be aware of the impact their *shopping behaviour* have on adequate bycatch protection of marine mammals. Various offcial certifcates have been introduced to ensure that labelled fsh was caught using techniques that minimise bycatch. Choosing a certifed instead of

..      **Fig. 3** Yellow acoustic deterrent devices, so-called 'pingers', are attached to a fshing net in order to minimise bycatch. Harbour porpoises are scared away by the sounds the pingers produce. © Annika Toth

an uncertifed product helps ensure that bycatch caused by traditional and more harmful fshing techniques will fnally be replaced by modern and marine mammal-friendly gear, limiting bycatch.

Seafood certifcation schemes span from self-certifcation to third-party ecolabelling schemes. Some of the most well-known certifcations of fsheries are the international *Marine Stewardship Council (MSC) and t*he *Friend of the Sea (FOS)*. Some countries have their own certifcations, such as the Swedish *KRAV* and German *Naturland*. Out of the 50 or more seafood ecolabelling schemes that are out there, MSC and FOS cover over 25% of the global seafood certifcations.

#### **Seafood certifcation schemes**

The *Marine Stewardship Council (MSC)* assesses if wild capture fsheries are sustainable and well-managed. MSC was established in 1997 as a partnership between WWF and the food company *Unilever*. A certain amount of marine mammal bycatch is accepted under the condition that it is sustainable and only has a small impact on populations. The certifcation is carried out by thirdparty certifers. About 15% of the world's fsheries are covered by MSC programmes. Assessments of fsheries is based on scientifc verifcation of the sustainability of targeted fsh stocks, the ecosystem impact and the quality of management of the fshery. In the future, MSC will adjust its assessment methodology to FAO (*The Food and Agriculture Organisation*, United Nations) guidelines for the ecolabelling of fsh and fshery products.

*Friend of the Sea (FOS)* was founded in 2006 by the Earth Island Institute's 'Dolphin Safe Project', which has been managing the 'Dolphin Safe' label. FOS is one of the most diverse seafood ecolabels and certifes both aquaculture and fsheries. The sustainable fshery criteria require no overexploitation of target stock, not more than 8% discards, no bycatch of endangered species, no impact on the seabed, compliance with regulations, social accountability and gradual reduction of the carbon footprint. The 'Dolphin Safe' label played a role in the reduction of dolphin bycatch in tuna fsheries. Despite these efforts, the affected dolphin populations have not recovered, which indicates that both fshery management and the 'Dolphin Safe' label are not effective enough. Dolphin-safe catch methods may be used by the industry primarily as a marketing tool rather than a genuine attempt to protect dolphin populations. Consumers should look for actual certifed labels on tuna cans rather than general 'dolphin friendly' prints. Also note that some dolphin-safe fshing practises can have a substantial bycatch of other threatened species of, for example, sharks and turtles.

The Swedish *KRAV* label for organic food has implemented a system for the certifcation of sustainable fsheries. Sustainability is evaluated using three criteria: safe fshing methods, sustainable stocks and *traceability*. Safe fshing methods require the use of fshing gear that eliminates capture of non-targeted species or undersized individuals. Sustainability ensures that fshing is carried out on stocks that can be maintained in the long term. Traceability allows checking the location of fshing vessels, ensuring they only fsh from approved stocks in authorised areas.

*Naturland* was founded in Germany in 1982 as an organic farming certifcation scheme. It later developed certifcation schemes for aquaculture and fsheries. Naturland standards on sustainable fshery focus on the careful use of fsh stocks while protecting entire ecosystems, avoidance of harmful fshing methods, and supporting fair working conditions for fshermen.

Seafood certifcation schemes are not without faws. Scientists and NGOs have been objected to some fsheries certifcations. For example, a substantial part of seafood certifed by MSC or FOS lacks stock status information. Target species are sometimes overfshed and therefore not worthy receiving the label. Some of the MSC certifers have been paid by the fsheries. Still, we believe it is sensible to purchase certifed seafood, as the fraction of less exploited or healthier fsh stocks is 3–4 times higher in certifed than in non-certifed products.

### **5 Whaling and seal hunting**

### **5.1 Whaling**

Whaling is the practice of hunting whales, dolphins or porpoises. In some parts of the world, whaling started at least 3,000 B.C., mainly as a food resource. In the 1800s, whales were also hunted for blubber (which was used for, lamp oil, lubrication and soap) and baleen (*whalebone*, used for, corsets and umbrella ribs). During the era of industrial whaling in the twentieth century, many larger whale species were hunted for meat to near extinction. The industrialised hunt had such an impact on populations that several species of large whales are still listed as endangered. Nowadays, whales are protected by several national laws and international conventions. The *International Convention for the Regulation of Whaling* was ratifed by many countries in 1946. Even though most countries have banned whaling, there are still a few countries hunting whales (. Table 2).

..      **Table 2** Examples of modern whaling operations, regulated by the IWC

#### **IWC and ASCOBANS**

The IWC (*International Whaling Commission*) manages whaling and is involved in cetacean research and conservation. The IWC regulates commercial, scientifc and aboriginal subsistence whaling. Membership of the IWC is open to any country that formally adheres to the 1946 Convention. In 1986, IWC implemented a *whaling moratorium* on large whales, pausing commercial whaling. The moratorium has given many whale populations a chance to recover from the extensive exploitation during the nineteenth and twentieth centuries.

The moratorium was affrmed in 2018 and is still effective. A few countries object to the moratorium and pursue whaling, targeting, for example, minke whales (*Balaenoptera acutorostrata*, e.g. Norway) and fn whales (*Balaenoptera physalus*, e.g. Iceland). In addition, some countries are allowed to pursue *scientifc whaling* to evaluate the status of certain populations. However, scientifc whaling has been suspected to be used as a 'loophole' to pursue commercial whaling under the moratorium. Japan is an important country in these discussions. After having conducted scientifc whaling sanctioned by IWC for decades, Japan resumed commercial whaling and left IWC in 2019.

*ASCOBANS (Agreement on the conservation of small cetaceans in the Baltic, North East Atlantic, Irish and North Seas)* was ratifed in 1991 and extended to its present extent in 2008. The objective of ASCOBANS is to restore or maintain a favourable conservation status for cetaceans in the area covered by the agreement. Working groups of scientists and conservationists within ASCOBANS develop area- or topic-specifc management strategies for the protection and recovery of local populations of marine mammals, such as harbour porpoises.

During the twentieth century, the era of industrial whaling, almost 3 million cetaceans were killed. Measured as total biomass, this constitutes one of the largest animal harvests in human history. In the North Atlantic, over 280,000 cetaceans were killed, in the North Pacifc 560,000, and in the Southern Hemisphere an estimated 2,000,000 animals. These are only the offcially documented numbers; the real numbers are likely much higher.

Once stocks of larger whales had been depleted, the whalers moved on to hunt other stocks, and eventually smaller species. Sailpowered whaling ships took around 300,000 sperm whales (*Physeter macrocephalus*) between the early 1700s and the end of the 1800s. Technological advances of the late nineteenth century made whaling extremely effcient. Engine-driven vessels and the *exploding harpoon* were introduced, as well as factory processing on huge ships or at whaling stations. In the frst 60 years of the 1900s, the same number of sperm whales were caught as during the previous two centuries. In the following decade (1970s), the same number of sperm whales were again harvested, this time due to a special interest in the waxy fuid inside their heads, called spermaceti.

### **5.2 Seal hunting**

Seal hunting also goes back several thousand years of human history. Seal bones and teeth are found in human-made deposits in Northern Europe from stone, bronze and iron ages. Seals have been hunted for fur, blubber and meat. They have also been hunted as pest control to reduce competition with fsheries.

During the past few hundred years, the two main target species for seal hunting in Northern Europe were the harbour and grey seal. In the late 1800s and early 1900s, a bounty system was established in several countries as an attempt to reduce seal stocks and their infuence on fshing. The bounty system led to an overexploitation of both species. With the additional detrimental effects of DDT, PCBs and other toxic substances, the populations plummeted in the 1950s and 1960s. Both grey and harbour seals have been protected from hunting since the 1980s in most Northern European countries. Since protection, the populations of Northern European seals have steadily increased, except for a few viral disease outbreaks with a high mortality in harbour seal populations in 1988 and 2002. Today, the abundance of both harbour and grey seals has reached a sustainable level. Still, the nutritional and health status of individual seals have not reached a satisfactory level.

Limited seal hunting (mainly for fshing gear protection, but in some areas also for food) is allowed in several Baltic countries, including Finland, Sweden and Denmark. In Germany, harbour seals belong to huntable game, but they have been exempted from this practice since 1974. Offcially appointed and trained seal hunters help the German authorities to collect dead animals, and to decide if seals in a poor health condition found alive on the beach should be sent for rehabilitation or *euthanised* with a rife.

### **Current topics of seal- fshery interaction research**

#### *Improved Fishing Gear*

Grey seals exhibit remarkable adaptability and cognitive abilities. In regions of high abundance, like the Baltic Sea, they locally create conficts with commercial fsheries by eating fsh from nets and destroying gear. To protect catch, fshermen may replace traditional fshing gear (such as gillnets) with alternative seal-safe gear (e.g. pontoon traps or cod pots). Despite the large progress that has been made in preventing seal-induced catch damage, none of these new types of gear are 100% seal safe. Seals quickly improve their skills in raiding fsh traps, and therefore constant modifcations of fshing gear are essential. It is necessary to continue the development of effective and sustainable seal-safe fshing methods, while at the same time prevent damage caused by seals. In order to improve fshing gear and to test new prototypes, a close cooperation between scientists, fshing gear technologists and fshermen is required.

At the Swedish *University of Agricultural Sciences* (SLU) in Lysekil, Dr. Sara Königson and her student, Jasmine Stavenow, investigate conficts between seals and fsheries. The aim of their research is to develop seal-proof fshing gear. In one of Sara's research projects, she has developed and tested pots as an alternative to gill nets used for cod. Sara and Jasmine attached waterproof cameras to the pots so that they could study both fsh and seal behaviour around the pots. They spent many hours on the pier or in small boats preparing the pots for the experiments, as well as in front of their computers, analysing the footage and summarising the results into reports and scientifc publications. Some of Sara's and Jasmine's seal-proof pots are now commercially available. Sara and Jasmine continue to invent or improve other types of fshing gear.

You can watch the video of their typical day of feld work and a video showing the behaviour of grey seals visiting fshing gear here . Videos S1 and S2.

### **6 Teaching materials**

### ?**Exercise 4.1: Fish dissection and otolith examination**

Did you know that bony fsh have *ear stones*, which can be used for species identifcation? How can you determine the age of a fsh?

Bony fshes have a sensory organ to detect gravity, balance and movement. Within this ..      **Fig. 4** X-ray image of a marine angelfsh showing the location of an otolith (indicated with an arrow). © Frédérich Bruno

organ, the fsh ear is also located. No external ear (such as our *pinna*) is necessary for fsh to hear underwater. Essential to fsh hearing is the *otoliths*, meaning ear- (oto) stones (liths). Each otolith is made of calcium carbonate crystals. Bony fsh have six otoliths, three on each side of the head in posterior end of the cranial cavity (. Fig. 4). Some of them are small, and usually only two of them (which in some species can grow very large, with a length of several cm) can be seen without a magnifying glass. When the fsh is rocked in a sound feld, the otoliths lag slightly behind due to their higher density, and the relative motion between fsh and ear stone is picked up by sensory hair cells.

Otoliths show *annual growth zones*, very much like growth rings on trees. Therefore, they can be used to age fsh. In herring and fat fsh, the annual growth zones can be seen without any kind of special preparation. In other species, such as cod, the otoliths must be prepared before the growth zones are visible. These otoliths are sectioned, polished or washed in weak hydrochloric acid solution in order to enhance the contrast between the different growth zones. Sometimes it is also necessary to break the otolith and burn the cracked surface with a burner or candle before age determination is possible.

There are many other applications to these stones besides age determination. Their toxic content may indicate ecosystem pollution. The growth layer in which toxins are detected may indicate at what age fsh encountered toxins. Otoliths are shaped differently depending on species, and they are therefore used for taxonomic studies and species identifcation. Even though otoliths found in marine mammal stomachs and faeces may be eroded by digestion, they still provide useful information on prey species. Such data can also enhance our knowledge on the magnitude of competition between wildlife and fsheries. Consequently, analysing otoliths makes it possible to study fsh, their predators, ecosystems, fshery interactions and environmental contamination.

In this exercise, you will learn how to remove otoliths from herring.

### z **Tasks**

1. Cut carefully through the top of the head with small scissors and expose the brain. The otoliths are located near the bottom of the brain case.

..      **Fig. 5** (7 Exercise 4.1) Otoliths of different fsh species. The black-and-white scale on top of each otolith is 5 mm long. Fish illustrations under public domain (CC0) © Wilhelm von Wright, Gervais and Boulart

### ?**Exercise 4.2: Whose scat is it?**

Have you ever wondered how scientists determine what aquatic predators eat? Why are dietary studies of marine animals important?

In this exercise you will investigate marine animal scat. You learn how scientists identify marine animals through the help of otoliths and other animal parts found in scat. This information is used to understand the feeding behaviour of marine mammals, their health status, their habitat range, the abundance of fsh and other animals within their local habitat, and much more. For example, in a study from 2013 the scientists used stomach contents to predict how much giant squid is eaten worldwide by sperm whales each year. The result was an astonishing 131 million individuals!

### z **Required materials**

### z **Tasks**

Your task is to identify the consumed fsh species by its otoliths and the bony fragments you found in the scat sample, and to identify the most likely predator by the prey remains present in their scat.

To identify the predator, here is some information on marine animal feeding preferences.  Did you ever consider why it is so diffcult to pass a ban on whaling, and why some people and nations are opposing it? Should whaling be continued in order to preserve cultural heritage? Can humans use whales as a source of food, just like we use many other types of animals?

Here, in a panel discussion, stakeholders from different interest groups with different opinions debate whether or not there should be a worldwide ban on commercial whaling.

There are very few countries that still pursue commercial whaling. Whale meat for human consumption is usually sold on local markets. The IUCN (*International Union of Conservation of Nature*), the worldwide authority on the status of nature, lists the minke whale, the major target species of whaling, as being of least concern. Countries that are objecting to the IWC's moratorium decision and establish their own catch limits must provide information on their catches to the IWC, while Japan is no longer obliged to report to the IWC as a non-member.

### z **Moderator**

You lead the panel discussion and examine the topic as comprehensively as possible. You should remain independent, neutral and not biased towards one opinion throughout the entire discussion, and you are in charge of controlling a fair distribution of the voiced contributions. You are also responsible for calling the different stakeholders to reason if the discussion gets too heated, and you should encourage active participation by more reluctant participants through direct questions.

To begin the discussion, you ask the participants to present themselves and to outline their position on commercial whaling. Watch the time during the introductions: Each participant only has 2-3 min. If you feel they would have needed more time to explain their views, you can always ask them questions during the debate.

Subsequently, you will start the discussion. Here are some ways to get started:

Make sure to engage all stakeholders in the discussion by giving alternative suggestions or asking about their opinion on that matter. If the discussion comes to a standstill, keep the conversation going. You may, for example:

### z **Stakeholders**

**Whaler:** Whaling is my job. I need to earn my money to feed my family. It is what I learned and what I grew up with. I don't have a university degree or much other working experience; I don't think I could provide for my family if I would have to stop this job. I don't understand why most people are so angry about my job. Compared to what humans do to other food production animals, the whaling I practice is much less harmful. Our modern methods guarantee a quick and humane death. Whaling is sustainable; there are plenty of whales out there in the ocean and the stocks keep growing. The whales have a good life, are free and happy and get killed without any notice. They had a better life and a better death than many animals raised for their meat in farms.

**Pro-whaling fsherman:** Fishing is our livelihood, and the whales eat too much fsh. We are not able to use our fshing quotas any longer due to whales. Fish remains our main export businesses. We need to reduce the whale population in order to fulfl our fshing goals. Some whales even eat fsh directly out of our nets!

**Anti-whaling fsherman:** Fishing is our livelihood, and the whales support us because they are ecosystem engineers. Many people believe that they eat fsh and thus compete with our catches, but that is not true. The larger whales eat plankton and small fsh that is not our target species. They defecate in the water, which enriches the ocean with nutrients and gives small plankton and krill food, which again serves as food source for larger creatures. Whales basically help the fsh—that we want to catch—grow. Additionally, whales that feed on the same species that we fsh always know where the highest fsh abundance is. We just need to go where the whales are and we will have good catches.

**Local politician:** This is a sovereign country making its own decisions. Foreigners do not rule it, and neither does IWC. We decide ourselves whether whaling should be carried out or not. Whaling has a long tradition here, and we are a traditionalistic country. We value our history and our ancestry. Additionally, whaling is sustainable. Why is it OK to slaughter millions of cows, pigs and chickens for food, but not a couple of happy, free-living whales? That is hypocritical.

**Whaling industry economist:** Whaling has a long tradition in this country. My family has been whaling for centuries. Whales have been an accessible, healthy and sustainable food source for decades. Additionally, it is worth a good amount of money to sell whale meat. There still is a demand on whale meat because people grew up eating it.  Therefore, our practice is sustainable and our takes are not leading to a population decline or extinction.

**The conservationist:** The world's oceans are vast. There is no international police force operating on the high seas. If commercial whaling is permitted to start up again, there will be no way to control international trade of whale meat and blubber. Each large whale might be worth hundreds of thousands of dollars. Illegal and unregulated whaling could once again drive the large whale species to the brink of extinction, just like it happened in the past.

**Animal welfare advocate:** We think that commercial whaling must be halted. No one knows how populations of whales will be affected by hunting, on top of other daily threats they face. Whaling is unethical, and whales are not suitable for human consumption. Whales live long lives and reproduce slowly. They cannot be killed in an ethical manner. Explosive harpoons often miss the right area on the whale's body, subjecting them to suffering a long, slow and painful death.

**Scientist:** Whales have been recognised as ecosystem engineers (their faeces enrich the ocean with nutrients, which feeds plankton and thus substantially supports the bottom of the food chain). Many whales have fxed migration routes, and we often notice that the same animal is mistaken for several. Therefore, populations can be misinterpreted as being larger than they really are. One species may be divided into subgroups, ecotypes or even completely different species, which we are not yet aware of and that we may exterminate through whaling. Additionally, whales are on top of the food chain, and with their extensive blubber reserves and their longevity, they accumulate all kinds of toxins. The accumulation of pollutants in stranded whales can be so high that they have to be disposed as toxic waste. Whaling nations give out warnings that pregnant women should not consume whale meat. This is clearly not healthy food.

**Citizen Robinson:** Only few citizens eat whale meat. I myself grew up never tasting whale meat and I have no ambitions in doing so. Whaling is an outdated tradition that should stop. Many tourists coming here don't like the fact that we are a whaling nation, and we probably have economic losses from people who decide to boycott this country due to our whaling activities. In my opinion, whaling harms our country by degrading its economy and reputation.

**Citizen Johnson:** Whaling is a tradition in this country and therefore we take a lot of pride in it. Many citizens of this nation support whaling because it is sustainable, healthy and good for the economy. Whale hunting creates jobs and provides a locally sourced food source. Also, whale meat is better than farmed meat, because the whale lived a happier life than a cow or pig. Why can't you have both whaling and whale safaris going on in the same country? In Sweden, people are hunting moose, and there are also moose parks where people can enjoy them.

**Tourism expert:** Tourism is one of the most important and fastest growing businesses in our nation. Tourists come to see our nature and also for whale watching. It is absurd that we keep killing whales, but the behaviour of tourists is as baffing to me. Some whale watching tourists even eat whale meat. But other tourists boycott our country as a holiday destination due to our whaling reputation.

### z **Tasks**

1. In this role play, the participants will assume characters of different parties involved in this confict of interest. Choose one of the potential roles (or have one assigned by your teacher) and collect arguments for your position in order to develop a basis for the discussion. Multiple students can form groups of parties. However, make sure that the number of stakeholders in each party of interest is equally distributed. Some additional reading material for more information can be provided by the teacher or obtained from the internet (e.g. the IWC and the IUCN websites). Prepare yourself for your stakeholder position and for possible *counter arguments*. One contestant (teacher or a student) is the moderator of the discussion and should be especially aware of the instructions given above for this role.

	- 5 What is your position on whaling?
	- 5 What are your main arguments?
	- 5 What compromise could be proposed in the interest of yourself or your organisation?

Choose one person of your stakeholder group to represent your party of interest during the discussion.

3. The moderator will start the discussion. Before the panel discussion begins, participants present themselves, their position and their arguments in 2–3 min. All participants are thus given the opportunity to get to know each other and their positions on the topic. After the introduction, the moderator will take the lead on the discussion between the different parties of interest and keep the debate alive.

4. In the end, all students should collectively summarise all pro-whaling and anti-whaling arguments and see if the class can come to a differentiated and objective consensus on their opinion on commercial whaling.

### **Suggested reading**

**Open Access** This chapter is licensed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license and indicate if changes were made.

The images or other third party material in this chapter are included in the chapter's Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the chapter's Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder.

# **Plastic Debris and Its Impacts on Marine Mammals**

*Katrin Kruse, Katrin Knickmeier, Dennis Brennecke, Bianca Unger, and Ursula Siebert*

### **Contents**

**Suggested reading – 61**

**Supplementary Information** The online version contains supplementary material available at [https://doi.org/10.1007/978-3-031-06836-2\_4].

	- 5 Understand plastic pollution problems in our oceans.
	- 5 Detect and quantify plastics in our environment and everyday products.
	- 5 Obtain knowledge-based ideas to reduce plastic pollution.

### **1 Introduction**

The *United Nations Environment Programme* defnes *marine debris* as "any persistent, manufactured or processed solid material discarded, disposed of or abandoned in the marine and coastal environment." Marine debris consist of many different materials, such as metal, glass, processed wood, and last but not least, plastics. Due to its durability and lightness, plastics make up a large share of all marine debris and can be found also in remote places.

"Plastics" refer to synthetic materials made primarily of crude oil. These materials have many interesting properties, such as lightness and durability. They can be hard, soft, elastic, persistent and formed in any imaginable way. These features make plastics usable almost everywhere. We have a hard time imagining our daily lives without plastics. Plastic materials are used for, for example, clothing and for food packaging and many other products. In the form of nylon, it is also used in fshing nets and fshing lines.

One problem with plastics is that they are not readily *biodegradable*. Together with the large quantities produced, as well as insuffcient ways of disposal and recycling, this means that we fnd plastics everywhere in the environment. In 2020, more than 360 million tonnes of plastics were produced globally. It is estimated that 4-13 million tonnes make their way into the oceans every year. Scientifc expeditions have revealed that more than fve billion pieces of plastic with a total weight of more than 250,000 tonnes are foating around in our oceans. This enormous amount of plastic has severe consequences for marine life. Animals become entangled in drifting garbage and fshing nets, and they confuse plastic items with food. Plastics which have been ingested by smaller animals is transferred to large animals by predation. Different size classes of plastics cause different threats. For marine mammals, studies of the impact of plastics have so far focused on larger debris causing entanglement or ingestion, whereas information on the impact of small plastic items is scarce.

### **2 Plastics: A problem with many sources**

There are many sources of marine plastic debris. A major input comes from land with rivers and beaches. Plastic waste from agriculture, industrial production, construction sites, or from people carelessly dropping waste accidentally ends up in the environment and is transported to rivers and beaches by wind and rain and end up in the oceans. Interestingly, 70% of global debris entering the oceans originate from only ten larger rivers.

Large amounts of plastic debris are mainly caused by poor waste management. But, even in regions with well-developed waste management, debris ends up in rivers and is then transported to the ocean.

Nearly 2.4 billion people (about 40% of the world's population) live within 100 km from the coast. In many countries, waste is still placed in large waste disposal sites that may be situated close to the sea. Strong winds blow large quantities of waste into the oceans. Many plastic items are only used once and then they are thrown away; this includes bottles, bags, cups, straws, and spoons. These items may end in the environment due to the carelessness of consumers or poor recycling systems or problematic waste disposal sites: Even if they are properly disposed in garbage cans, they may still end up in the oceans.

Every time you wash your clothes, thousands of plastic fbers are released into the wastewater. The fbers are too small to be fltered out of wastewater at treatment facilities and therefore make their direct way to the ocean.

Thousands of ships and boats navigate our seas and rivers. Shipping containers may be lost at sea and—although it is nowadays strictly prohibited—rubbish may be thrown overboard. Sea-based sources, including shipping and offshore industry, contribute with 20% of marine debris. During fshing operations, nylon nets, lines and ropes are accidentally lost or dumped at sea. Abandoned fshing nets may stay active catching fsh for a long time period: They are known as *ghost nets*. *Dolly ropes* are orange or blue plastic threads that are used to protect bottom trawling nets against wear and tear. Every year, thousands of kilos of dolly ropes end up in the sea.

### **3 Where does plastics end up, and what are the consequences for marine life?**

Oceanic water is always in motion. Water is transported around by the huge currents that link all ocean basins. Surface and deep ocean currents belong to the same transportation network that work a bit like conveyor belts move water around the world. The atmosphere and the ocean currents infuence one another. Some ocean currents form giant gyres spanning several hundred, even thousands of kilometers in diameter. Once entering the oceans, marine debris circulates in these current systems for decades, perhaps even for centuries, and get concentrated within *gyres* created by the currents.

In 1997, scientists discovered a large area with marine debris in the North Pacifc, named the *Great Pacifc Garbage Patch*. The total weight of all man-made objects in this area was estimated to be 80,000 tons, or the same weight as 500 jumbo jets.

It may take hundreds of years for plastic to degrade. Plastic items are disintegrated into smaller parts by wind and sun. Older objects become brittle and then fragment into smaller and smaller pieces ending at so-called microand nanoplastics. Plastic objects either sink to the seabed or keep foating at the sea surface or within the water column. The weight of foating plastics is enhanced when colonized by small organisms, such as barnacles, mussels, and bacteria. This promotes the sinking process.

The ocean currents transport plastics and its inhabitants over large distances into new regions. Bryozoans, barnacles, hydroids, and molluscs hitch-hike on plastic litter. For *invasive species*, they may reproduce more rapidly in a new area and outcompete native species. This can change the composition of ecosystems and interfere with existing food chains.

During the past decades, scientists have observed impacts of plastic ingestion or entanglement in more than 550 species of marine animals, ranging from invertebrates to mammals. Marine mammals, turtles and seabirds become entangled in discarded fshing gear, restricting the animals in their movements, as well as in feeding or reproduction activities (. Figs. 1 and 2). Marine mammals entangled in submerged and anchored "ghost nets" are unable to surface for breathing and drown. A fshing rope tangled at the base of a tail of a whale can cut off blood circulation and eventually the entire tail.

In the North and Baltic Seas, marine debris cause severe health problems for wildlife. Marine mammals such as harbour porpoises, harbor seals and grey seals have shown a variety of impacts associated with entanglement and ingestion. This includes infammation of organs and suffocation. There has been a drastic rise in the number of reported events during the past decades. Still, there is no satisfactory understanding of the signifcance of marine debris for marine mammals.

Another problem is that many animals mistake plastic fragments with food. By eating plastic fragments, animals may acquire a full stomach without gaining any nutrients—they can starve. Scientists investigated sperm whales which accidentally ended up in beaches along the North Sea in early 2016. In 9 out of 30 whales, a variety of plastic debris was found in their stomachs. The animals had engulfed nets, ropes, foils, packaging material, and even a large plastic part from a car engine (. Fig. 3). One animal was found with 24 kg of marine debris in its stomach.

..      **Fig. 1** A humpback whale caught in a net. The whale was completely covered with the net and one of its pectoral fns was stuck. Several attempts were needed

before the whale could be released. Many whales which cannot be helped die—often because they are no longer able to reach the surface to breathe. © Gary Freitag

..      **Fig. 2** Gannets at a breeding colony on the island of Helgoland. Many Gannets are found dead due to entanglement in net fragments. © Abbo van Neer

..      **Fig. 3** (**a**) A fshing net found in a stomach from a sperm whale, stranded in the North Sea. (**b**) The net was 13 m long.  Large amounts of marine debris were found in sperm whales stranded along the North Sea coast in early 2016. Marine Pollution Bulletin 112(1–2): 134−141

In 2018, a dead young male sperm whale was found with 29 kg of plastic trash off the coast of Spain. A necropsy revealed that the animal had ingested, for example, trash bags, polypropylene sacks, ropes, and net segments. Some months later, in 2018, another sperm whale was found washed ashore in Indonesia. Scientists found hundreds of plastic items in its stomach, including cups, bags, and sandals. In 2019, scientists opened the stomach of a young dead whale in Sicily and found many plastic bags inside. In 2019, in the island of Mindanao, Philippines, a young Cuvier's beaked whale was found having 40 kg of plastic bags, including 16 rice sacks, in its stomach.