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, whitetip and blacktip 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.
“These types of surveys can vastly overcount numbers of large mobile fishes (such as sharks),” one of the paper’s co-authors, Julia Baum, an assistant professor at British Columbia’s University of Victoria, wrote in an e-mail.
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,” Shivji wrote in an e-mail.
The study showed both the potential conservation benefits, and limits, of creating marine reserves in remote areas. Several of the areas the researchers surveyed — including the northwestern Hawaiian Islands, three Mariana Islands and all of the islands in a region known as the Pacific Remote Island Area — enjoy a significant level of federal protection. Enforcement, however, is often absent.
Baum wrote in an e-mail that she regularly sees a large fishing vessel in U.S. waters near Kiritimati atoll in the northern Line Islands while conducting field work, and this operation hires local villagers to cut fins from sharks.
“To me, enforcement of these islands is a major unsung conservation challenge, and I suspect that if this is not effectively addressed [as soon as possible], the reef sharks on these islands will be fished out within the next 10 years,” she wrote.