The Fall of the Reef Shark
Sharks have a reputation of being apex predators of the sea. But even they have their weak points. Many shark populations have plummeted in the past three decades as a result of excessive harvesting — for their fins, as an incidental catch of fisheries targeting other species, and in recreational fisheries. This is particularly true for oceanic species. However, until now, a lack of data prevented scientists from properly quantifying the status of Pacific reef sharks at a large geographic scale. Curious gray reef sharks at Kure Atoll in the Papahanaumokuakea Marine National Monument, Hawaii were studied as part of a study published April 25 in the journal Conservation Biology. An international team of marine scientists provided the first estimates of reef shark losses in the Pacific Ocean using underwater surveys conducted over the past decade across 46 US Pacific islands and atolls, as part of NOAA's extensive Pacific Reef Assessment and Monitoring Program. The team compared reef shark numbers at reefs.
It is estimated that 100 million sharks are killed by people every year, due to commercial and recreational fishing. Sharks are a common seafood in many places, including Japan and Australia. In the Australian state of Victoria, shark is the most commonly used fish in fish and chips, in which fillets are battered and deep-fried or crumbed and grilled.
"We estimate that reef shark numbers have dropped substantially around populated islands, generally by more than 90 percent compared to those at the most untouched reefs", said Marc Nadon, lead author of the study and a scientist at the Joint Institute for Marine and Atmospheric Research (JIMAR) located at the University of Hawaii, as well as a PhD candidate with Dr. Jerry Ault at the University of Miami's Rosenstiel School of Marine & Atmospheric Science.
To obtain these estimates, Nadon and his colleagues used an innovative survey method, called towed-diver surveys, which were designed specifically for the census of large, highly mobile reef fishes like sharks. The surveys involve paired SCUBA divers recording shark sightings while towed behind a small boat.
"Towed-diver surveys are key to our effort to quantify reef shark abundance," said Ivor Williams, head of the team responsible for these surveys. "Unlike other underwater census methods, which are typically at an insufficient spatial scale to properly count large, mobile species, these surveys allowed our scientists to quickly record shark numbers over large areas of reef."
The team crunched the numbers from over 1,600 towed-diver surveys, combining them with information on human population, habitat complexity, reef area, and satellite-derived data on sea surface temperature and oceanographic productivity.
The models showed the enormous detrimental effect that humans have on reef sharks.
"Around each of the heavily populated areas we surveyed — in the main Hawaiian Islands, the Mariana Archipelago, and American Samoa - reef shark numbers were greatly depressed compared to reefs in the same regions that were simply further away from humans." Nadon said. "We estimate that less than 10% of the baseline numbers remain in these areas."
"The pattern — of very low reef shark numbers near inhabited islands — was remarkably consistent, irrespective of ocean conditions or region," added Williams.
"Our findings underscore the importance of long-term monitoring across gradients of human impacts, biogeographic, and oceanic conditions, for understanding how humans are altering our oceans," concluded Rusty Brainard, head of the Coral Reef Ecosystem Division at NOAA's Pacific Islands Fisheries Science Center, which conducted the surveys.
For further information see USGS or Sharks.
Reef Shark image by P. Ayotte via NOAA.