Global analysis reveals how sharks travel the oceans to find food

Monday, 29 January 2018

A University of Newcastle researcher is part of a major international collaboration that has revealed a greater insight into the feeding habits of the world’s most misunderstood fish – the shark.

sharks feeding

The collaboration, led by the University of Southampton, could help global efforts to overturn recent declines in the world’s shark population, which includes more than 500 known species.

The study, involving 73 scientists from 21 countries including the University of Newcastle’s Dr Vincent Raoult, used chemical markers in the form of carbon isotopes found in sharks to investigate where in the world they have been feeding – an unresolved question for many shark species.

The research revealed that sharks living near to the coast feed locally across a range of different food webs – similar to people living in a city who have access to a variety of different restaurants in the neighbourhood and no need to travel far to find the food they want.

On the other hand, oceanic sharks that are found throughout the world’s oceans appear to get most of their food from specific areas of cooler water in the northern and southern hemispheres. This is more like travelling long distances from rural areas to spend lots of time eating in a few restaurants in a distant city.

Dr Raoult provided data from two species of sawsharks caught in Tasmania as part of another project.

“This research really highlights how highly connected our oceans are, but also the wide-ranging impacts that shark fisheries may have: sharks caught in one area might spend large amounts of time in another, and therefore catching those sharks might have ecological impacts that traverse national boundaries and oceans.

“It also highlights how little we know about deep-ocean sharks, since most of the data for those species was from the North Atlantic.”

All life depends on carbon at the bottom of the food chain. Carbon comes in three forms or isotopes, and the proportions of two of the most common isotopes vary across the world’s ocean. The study compared the carbon isotopes from more than 5000 sharks from 114 species across the globe with those from phytoplankton at the bottom of the food web.

Led by Dr Christopher Bird at the University of Southampton, the study, published in Nature Ecology & Evolution, helped identify important shark feeding grounds.

“If an animal feeds in the same place where it was caught, the carbon isotope signals in the shark and phytoplankton will match,” Dr Bird said.

“However, if the shark has moved between feeding and where it was caught, then the signals will be different.

“You’ve heard of ‘you are what you eat’ - well this is more ‘you are where you ate’. We were able to show that sharks living close to land and those that live in the open ocean have very different ways of feeding,” Dr Bird said.

Senior author Dr Clive Trueman, Associate Professor in Marine Ecology also from the University of Southampton said globally, sharks were not doing well.

“Many shark populations have declined in the last few decades, particularly in the wide-ranging oceanic sharks that are targeted by fishing boats and caught accidentally in tuna fisheries as ‘by-catch’.

“Governments are now creating large marine protected areas around the globe, which help to reduce fishing, but most of these protected areas are in tropical waters, and may not provide effective protection for oceanic sharks.

“Sharks urgently need our help, but to help them we also need to understand them. Our study has helped by identifying important shark feeding grounds. New technologies like satellite and isotope tracking are giving us the information we need to turn the tide on these beautiful and fascinating animals.”

Dr Raoult said study would not have been possible were it not for the global collaboration involving more than 70 researchers.

“This is one of the rare studies that has the ability to inform global conservation guidelines regarding the geographic patterns of sharks.

“More accurate information about which areas of the global ocean are shark feeding grounds will help design more effective conservation measures.”

The paper ‘A global perspective on the trophic geography of sharks’ is published in the February issue of Nature Ecology & Evolution.

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