Sharks have gone from bycatch to target catch

Oceans are vast, but it appears that for sharks there may be few places left to hide from another top predator—longline fisherman. Scientists have long tracked the movements of sharks, finding out where they go to hunt, but until recently they had never tracked them as prey. In a study just published in the Proceedings of the National Academy of Sciences, however, an international team of scientists harnessed tracking technologies to monitor the fine scale movements of both sharks and fishing fleets over multiple years, finding a jaw-dropping 80 percent overlap between the two. The study also revealed that this fishing pressure never lets up throughout the year, as fleets move along with sharks as they migrate seasonally.

The research was conducted in the North Atlantic, where the most long line hooks are put in the water. It’s no surprise that sharks are killed as incidental bycatch, since they share a predilection with longline fishing vessels for high-productivity ocean fronts. But according to co-author David W. Sims, in recent years, as tuna and swordfish stocks have declined and their catches have been restricted by quotas, sharks have gone from bycatch to target catch for longliners, who can make up for lost income by selling their fins to Asian markets.

“If they aren’t targeting sharks, our finding of an 80 percent overlap is pretty amazing,” says Sims, a professor at the University of Southampton in the UK. “Longliners in the Atlantic are so reliant on sharks to balance their books that the idea that there is no regulation controlling their catches seems absurd.”

Management of sharks has been complicated, Sims says, by a lack of high-quality data on sharks abundance and landings.

Sims and his coauthors satellite tracked nearly 100, tagged, blue, shortfin mako, tiger, and hammerhead sharks and poured over nine years worth of GPS data from the Vessel Monitoring Systems of 189 Spanish and Portuguese longline fishing vessels, two of the most important fleets in terms of shark catches. Their analysis focused principally on the blues and makos, which account for 95 percent of shark landings. When they compared the data, researchers found that not only was there a spatial overlap for these two species, but a temporal one as well. In high use areas, blue sharks were close enough to fishing vessels to be at risk of capture 20 days per month and mako sharks 12 days per month.

Moving forward, Sims says the researchers are going to go global with their tracking efforts and collect data from all vessels on the high seas. But this study alone, he says, makes a compelling case for the introduction of international catch quotas to manage the shark fishery. — Catherine Elton | 5 February, 2016

Source: Queiroz, N., et al. 2016, Ocean-wide tracking of pelagic sharks reveals extent of overlap with longline fishing hotspots. PNAS. DOI: 10.1073/pnas.1510090113

Header image: courtesy Marine Biological Association