Commercial fishing to blame for planet's declining shark numbers

Shark populations are shrinking across the globe and new research suggests commercial fishing is to blame.

Scientists with the Zoological Society of London analyzed shark population dynamics in ecosystems throughout the world's oceans. The data suggests shark numbers are lowest in areas closest to population centers and large fish markets.

Sharks aren't just less abundant in places featuring significant commercial fishing activity, they're also smaller.

The data showed marine predators are smaller and less abundant within 750 miles of populations with more than 10,000 people. The new analysis -- detailed this week in the journal PLOS One Biology -- showed sharks are also smaller in warmer waters.

"Human activity is now the biggest influence on sharks' distribution, overtaking every other ecological factor," Tom Letessier, researcher with the ZSL's Institute of Zoology, said in a news release. "Just 13 percent of the world's oceans can be considered 'wilderness' but sharks and other predators are much more common and significantly larger at distances greater than 1,250 kilometers from people."

The presence of human populations and commercial fishing fleets makes it difficult for sharks and other marine predators to thrive. Some predators are accidentally caught and killed, while fishers in some parts of the world purposefully target sharks. Because sharks and commercial fishers often target the same prey, sharks struggle in places where industrial fishing has depleted commercial fish stocks.

Scientists measured shark size and abundance by surveying thousands of video observations recorded at test sites all over the world. The baited cameras recorded 841 individual shark visits, comprising 19 different species.

"Our study also found that shallower water habitats, of depths less than 500 meters, were vital for marine predator diversity," Letessier said. "We therefore need to identify sites that are both shallow and remote and prioritize them for conservation. However, there are still numerous shallow hot spots in the vicinity of human markets that are not appropriately protected, and this must change. Existing large, no-take MPAs [marine protected areas] need to be better enforced and extended to focus on the last refuges where these extraordinary animals remain abundant.

"Large marine predators -- and sharks in particular -- play a unique and irreplaceable role in the ocean ecosystem," he said. "They control populations of prey species, keep those populations healthy by removing sick or injured animals, and transport nutrients between loosely connected habitats over vast distances."

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