Shark hotspots under worldwide threat from overfishing

Using this tracking information, researchers identified areas of the ocean that were important for multiple species, shark "hot spots," that were located in ocean frontal zones, boundaries in the sea between different water masses that are highly productive and food-rich. Researchers then calculated how much shark hotspots overlapped with global longline fishing vessels -- the type of fishing gear that catches most open-ocean sharks.

The study found that 24% of the space used by sharks in an average month falls under the footprint of longline fishing. For commercially exploited sharks such as blue and shortfin makos sharks in the North Atlantic, the overlap was much higher, with on average 76% and 62% of their space use, respectively, overlapping with longlines each month. Even internationally protected species, such as great whites, had over 50% overlap with longline fleets.

"Our results show major high seas fishing activities are currently centered on ecologically important shark hotspots worldwide," says Professor David Sims, who led the study as part of the Global Shark Movement Project based at the Marine Biological Association Laboratory in Plymouth, UK.

Equally alarming was that shark hotspots showing high overlap with longline fishing were often also subjected to high fishing effort, a potential 'double whammy' for sharks that will result in higher catch rates and potentially accelerate declines in abundance.

"Some shark hotspots were exposed to higher than average fishing effort for as much as half the year," said Nuno Queiroz, a lead researcher in the study from the University of Porto in Portugal.

The team's findings indicate large sharks -- some of which are already endangered globally -- face a future with limited spatial refuge from industrial longline fishing effort.

"Currently, little to no protection exists for sharks in the high seas. It's clear from our study that immediate conservation action is needed to prevent further declines of open-ocean sharks," said Neil Hammerschlag, a study co-author and research associate professor at the University of Miami Rosenstiel School of Marine and Atmospheric Science. "These finding are concerning because as top predators, sharks help maintain healthy ocean ecosystems," said Hammerschlag.

Hammerschlag provided data from 154 tagged sharks of 10 species which included blacktip, blue, bull, Mako, scalloped hammerhead, sandbar, silky and tiger.

The researchers propose that designated large-scale marine protected areas around regions of shark activity could be one solution. The study suggests that the detailed maps of shark hotspots overlapping with longline fishing provide a 'blueprint' for deciding where to place large-scale marine protected areas (MPAs) aimed at conserving sharks, in addition to the need for strict quotas to reduce catches elsewhere.

"Some of the shark hotspots we studied may not be there in as little as a few years' time if management measures are not put in place now to conserve the sharks and the habitats on which they depend," Sims says.