DANGER: Sharks at risk in this area
A global study of sharks has found that they’re in danger of being fished over a quarter of their entire habitat. This comes from an international collaboration of marine biologists, who have tracked ocean sharks to produce the first ever world map of their hotspots. By comparing the hotspots to fishing routes, they have shown that the most valuable species are in danger over almost three-quarters of their entire range. Phil Sansom spoke to David Sims, one of the study’s authors, who is a senior research fellow at the Marine Biological Association Laboratory in Plymouth.
David - Pelagic means "of the open ocean," and so pelagic sharks are sharks which roam the open ocean. By using satellite tracking to identify shark hotspots across the global ocean and also tracking longline fishing vessels, we found that the overlap of the shark hotspots where they prefer to be and hangout, overlaps at least 24% of the time with these longline fishing vessels.
Phil - So that's like a quarter of the entire area that sharks live in is an area that they’re in danger of being fished?
David - We couldn't track every shark, but for the just under 2000 sharks that we did track yeah, 24% of the places they prefer to gather and to be, were entirely overlapped by longline fishing vessels. And these longline fishing vessels, each of those vessels deploys a line that's a hundred kilometres long with over a thousand baited hooks.
Phil - So that's a serious deal for sharks, right?
David - It is, yeah. Obviously, they're attracted to the baits on these lines, and because the lines are so long and there are thousands of these vessels across the global oceans which are trying to catch sharks, because, as opposed to 20 odd years ago sharks have a real value now.
Phil - Now let me ask you; it can't be easy to figure out where sharks gather or where they like to live, how did you actually do it?
David - A project at a global scale requires a global scale research group. There's 150 scientists across 100 institutes in 26 countries. We tagged just over 1800 different pelagic sharks and we gathered all the satellite tracking data of all sorts of different species of pelagic sharks. We gathered it all together in a giant database and then we were able to analyse, to explore, the movements of the sharks in relation to the environment.
And what we found was that the sharks prefer to be in areas where there are strong boundaries between different water masses with different characteristics, so fronts between different temperature water masses for example. And these sort of transition areas, these sort of boundaries are areas where there's usually lots of plankton and, of course, that plankton attracts in fish and squid that the sharks like to feed on.
Phil - That's so interesting that you actually had to physically tag the sharks.
David - That's it. That's the nature of this project is that it's the first one that's really worked at the global scale for pelagic sharks. We tracked about 23 different species, including the Great White shark and three species of hammerheads actually, but also the Blue shark and Shortfin Mako shark, and they're interesting because those two species make up about 90% of the pelagic sharks which are caught by high seas and shell fisheries. So those two are the most commercially important and obviously they're the ones that we are particularly concerned about as a consequence of this study, because we found that, in fact, 76% of blue shark space use and 26% of shortfin mako shark space use was entirely overlapped by these longline fishing boats.
Phil - Now let me ask you because you keep saying "overlap," but how do you know that overlap necessarily means they're in trouble?
David - Yeah, it's a really good question. We actually thought about that and in the study we actually calculated the fishing effort that was going on and related that fishing effort to actual landings, so these are tons of sharks that have been recorded on the global databases. And what we found was a positive relationship between the amount of fishing effort and the magnitude of landings of those species, so there was a direct link between the fishing effort, the overlap, and mortality.
Phil - So what do we do?
David - What we propose is that the maps we’ve produced of where the hotspots are can really start to become the foundational blueprint of shark conservation at the global scale. And so where we have these shark hotspots it could be that these are the sites that policymakers and scientists may select to protect sharks. That's not to say that that's the only tool that can come from this sort of study.
One thing that this does suggest is that the surveillance of megafauna like sharks is actually a very powerful way in which you can start to look at a new management of the oceans. If you know where the sharks are and maybe other megafauna like turtles and whales, it might be possible then to focus where the enforcement happens and for focusing where particular patrol vessels might be, for example. So I think the future holds this much more broad and satellite-based technology led conservation, which I think for sharks will be absolutely crucial.