Protecting the big fish in the sea

By Lynley Hargreaves 19/11/2014 1

Dr Michael Plank
Dr Michael Plank

Throw the small fish back, so they can grow and reproduce. So goes conventional scientific wisdom, as well as most of the world’s fishing regulations. But research – including Marsden-funded mathematical models by the University of Canterbury’s Michael Plank – is now showing that we should be catching more small fry, and letting the big fish go free.

How did you get involved in this research?

We were interested in a way of modelling fish populations from an academic point of view. The idea of balanced harvesting was put forward in an article in Science and we realised we had a model we could test it with. The result was not really what we expected, but when you stop and think about it, it makes a lot of sense. That’s the power of mathematics, giving you insight into why you are seeing the results that you are. That was the first time that a group had combined balanced harvesting with a model that keeps track of how biomass flows through the population, from prey to predator to offspring.

What is balanced harvesting, and what are the benefits?

It means harvesting fish according to their natural productivity. Small fish tend to have higher productivity than big ones because they grow faster and there are more of them, so in practice balanced harvesting means catching more small fish. This allows you to net bigger catches, in the same way that eating plants is more efficient than eating meat, because it’s lower down the food chain. An adult fish can produce millions of eggs over its lifetime, so it’s worth protecting big fish as it helps ensure a sustainable population.

We’ve been targeting big fish. What has that done to our oceans?

It has massively depleted the numbers of large adults, with knock on effects for the rest of the ecosystem. It has even caused fish to evolve to mature at smaller sizes. For example, adult Northeast Arctic cod are now almost 2 kg smaller than they were 70 years ago.

What size fish should fishers be catching?

At Te Pūnaha Matatini, we are investigating what would happen if fishers were free to target whatever size they wanted. Would their catches go up or down and would it be sustainable? The best size to catch depends on the species and it’s difficult to model species in isolation because they influence each other. We are working towards a model that looks at lots of species together, which is what we need to really understand the best way to fish sustainably at the same time as feeding a growing population.

Is this happening anywhere in the real world, and is it likely to happen in New Zealand?

There are examples of small-scale fisheries in Africa that catch small fish, yet are both high-yielding and sustainable. In New Zealand, anyone who’s eaten whitebait knows that small fish can taste good; and traditional Māori practice has often targeted the small and thrown back the large. Perhaps there are things we can learn from this when managing commercial stocks here.

These interviews showcase researchers supported by the Marsden Fund which, since 1994, has been supporting fundamental, investigator-led research in New Zealand.

One Response to “Protecting the big fish in the sea”

  • Parenthetically, this is similar to how sustainable harvest of crocodilians works. The shift away from the very destructive harvests of the early-mid 20th C was achieved by replacing adult harvest, with juvenile (egg) harvests.

    They have a much faster growth rate than adults, and a much higher natural mortality- which dampens a lot the impact of their removal.