Decline of Pacific reef sharks tied to human fishing pressure

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Pacific reef shark populations have plummeted by 90 percent or more over the past several decades, according to a new study by a team of American and Canadian researchers, and much of this decline stems from human fishing pressure.

Quantifying the decline for the first time, the analysis, published online Friday in the journal Conservation Biology, shows that shark populations fare worse the closer they are to people, even if the nearest population is an atoll with fewer than 100 residents.

The team of eight scientists examined the results of a decade of underwater surveys across 46 Pacific islands and atolls and found densities of reef sharks -- gray, white-tip and black-tip reef sharks, as well as Galapagos and tawny nurse sharks -- "increased substantially as human population decreased" and the productivity and temperature of the ocean increased.

"Our results suggest humans now exert a stronger influence on the abundance of reef sharks than either habitat quality or oceanographic factors," the authors wrote.

Near populated places, such as the main Hawaiian islands and American Samoa, the study found, there were roughly 26 sharks per square mile. Remote reefs, such as in the Northwestern Hawaiian Islands and Johnson Atoll, a U.S. territory west of Hawaii, by contrast, boasted 337 sharks per square mile.

"In short, people and sharks don't mix," Marc Nadon, the study's lead author and a scientist at the University of Hawaii's Joint Institute for Marine and Atmospheric Research, said in a statement.

The scientists relied on more than 1,600 "towed-diver surveys" for their study. This form of underwater survey, aimed at reaching a more accurate count of fast-moving, wide-ranging fish, entails having a pair of scuba divers record the number of sharks they see while being towed behind a boat.

The researchers said previous underwater surveys, which focused on a small transect of the ocean or a stationary point, skewed results by double-counting some sharks that passed through the same area multiple times.

Mahmood Shivji, who directs the Guy Harvey Research Institute at Nova Southeastern University in Florida, said the new paper's "results are consistent with other studies showing a decline in reef shark numbers elsewhere."

A 2010 study by Australian and British researchers, for example, showed that reef shark populations had declined 90 percent since the 1970s at three remote atolls in the Chagos Archipelago in the Indian Ocean.

"In this case, even though the atolls have few people, the decline is attributed to distant-origin fishing fleets," Mr. Shivji said by email.


First Published April 30, 2012 12:00 AM


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