As the basis of all food webs, carbon can be traced in different forms called isotopes. The ratios of carbon-13 isotopes in plant tissue will be different according to the type of plant photosynthesis taking place. This means that the ratios of carbon-13 will differ in phytoplankton in oceans around the globe. Scientists can use these chemical tracers to ascertain which types of plants were eaten by animals. Where the isotopes have passed along the food chain and left a signature in sharks, they can also determine where the sharks were feeding. ‘If an animal feeds in the same place where it was caught, the carbon isotope signals in the shark and phytoplankton will match,’ said Bird in an interview for the University of Southampton’s website. ‘However, if the shark has moved between feeding and where it was caught, then the signals will be different.’
After measuring isotopes from 114 different shark species around the world, the researchers found that sharks of the continental shelf feed close to home across a range of food webs. Protecting them using marine protected areas (MPAs) could well be a suitable conservation strategy, provided that the MPAs cover the range of habitats used. Conversely, oceanic sharks seem to travel to mid-latitudinal regions to feed in cooler waters where nutrients are rich – but the competing presence of fisheries poses a conservation threat, particularly where oceanic sharks are vulnerable to exploitation.
In the same interview with Southampton University, co-author Clive Trueman asserted, ‘The results have important implications for conservation. Globally, sharks are 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.’
