Study: Choosing Fish May Be Killing Sharks

(CNN) -- The ideal shark hotspot in the ocean is rich in appealing food -- and that's exactly where commercial fishing vessels are snagging sharks along with the fish they catch, according to a new study.

An international team of researchers spanning 26 countries tracked about 2,000 sharks using satellite tags to determine areas in the ocean where multiple shark species converged. They were found in ocean frontal zones, which act as the boundaries between different water masses. Nutrients and plankton flourish here, which encourages fish to flock to the zones.

While this feeding ground draws sharks, it also draws commercial longline fishing vessels. This method can include hundreds of baited hooks hanging from a single line used to catch swordfish, tuna and halibut. But they also catch dolphins, sea turtles, birds and sharks.

The movement data of the sharks helped the researchers determine that one quarter of the sharks' habitats were also located in active fishing areas. The study published July 25 in the journal Nature.

"Our results show major high seas fishing activities are currently centered on ecologically important shark hotspots worldwide" said David Sims, study author and team leader with the Global Shark Movement Project based at the Marine Biological Association Laboratory in Plymouth, UK.

Oceanic sharks migrate across vast parts of the ocean, and they also account for about half of the sharks pulled in by fisheries. Some of the shark hotspots also overlap in areas where a higher than normal concentration of fishing occurs, which causes more sharks to be caught.

"Some shark hotspots were exposed to higher than average fishing effort for as much as half the year," said Nuno Queiroz, study co-author and researcher at the University of Porto's Center in Biodiversity and Genetic Resources.

Blue sharks and shortfin mako sharks are also actively exploited for leather, oil and food, so their space overlaps with longline fishing between 62% and 76%.

And although great white sharks are protected, their populations overlapped 50% with commercial fishing.

"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 findings are concerning because as top predators, sharks help maintain healthy ocean ecosystems."

The researchers believe a map of hotspots could help determine areas that should be declared as protected to help conservation efforts, as well as quotas to reduce catches.

"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 said.

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