Researchers say in certain hotspots of shark activity it may be necessary to introduce catch quotas or size limits to prevent overfishing. (Credit: iStockphoto)

Satellites find spots where sharks may be overfished

While catch rates have declined significantly for many fish species, oceanic shark fishing remains largely unregulated and tens of millions of ocean-dwelling sharks are caught each year.

Further, researchers say a lack of data on where sharks are likely to encounter fishing vessels hampers conservation efforts.

“In the North Atlantic, it seems threatened sharks have few places left to hide in the face of industrialized, high-seas fishing of the last 50 years.”

“Many studies have tracked sharks, and many studies have tracked fishing vessels, but fine-scale tracking of sharks and fishing vessels together is lacking, even though this should better inform how shark fisheries should be regulated,” says David Sims, a professor at the University of Southampton.

To investigate the issue, scientists tracked more than 100 sharks from six different species by satellite across the entire North Atlantic, one of the most heavily exploited oceans. Concurrent with the shark tracking, they tracked 186 Spanish and Portuguese longline fishing vessels using GPS to quantify the overlap in space and time.

From the sharks’ satellite tracks and from remote sensing images of the ocean environment, researchers found that within each species’ preferred range, sharks tended to aggregate in locations characterized by strong temperature gradients and high productivity.

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Researchers say that while the fact that fishing vessels targeted the same area as the sharks was not a surprise, the scale of the overlap was.

For the most heavily fished shark species, blue and mako, about 80 percent of the sharks’ tracked range overlapped with the fishing vessels’ range, with some individual sharks remaining near to longlines for more than 60 percent of the time they were tracked.

“Although we suspected overlap might be high, we had no idea it would be this high,” says lead author Nuno Queiroz of the University of Porto, Portugal. “Space-use overlap on this scale potentially increases shark susceptibility to fishing exploitation, which has unknown consequences for populations.”

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Researchers say in certain hotspots of shark activity it may be necessary to introduce catch quotas or size limits to prevent overfishing. Areas of highest overlap included the Gulf Stream, the North Atlantic Current/Labrador Current Convergence Zone near Newfoundland, and along the Mid-Atlantic Ridge southwest of the Azores.

Over the period 2005-2009, the overlap between sharks and fishing vessels persisted between years in hotspot locations.

“This highlights how broadly the fishing exploitation efficiently ‘tracks’ oceanic sharks within their hotspots year-round, Sims says. “In the North Atlantic, it seems threatened sharks have few places left to hide in the face of industrialized, high-seas fishing of the last 50 years.”

Their findings appear in the Proceedings of the National Academy of Sciences,

Source: University of Southampton