Overfishing large sharks

hammerhead_010705

Fewer big sharks in the oceans mean fewer bay scallops and other shellfish, according to an article in the journal Science, tying two unlikely links in the food web to the same fate.

A team of Canadian and American ecologists, led by world-renowned fisheries biologist Ransom Myers at Dalhousie University, has found that over fishing the largest predatory sharks, such as the bull, great white, dusky, and hammerhead sharks, along the Atlantic Coast of the United States has led to an explosion of their ray, skate, and small shark prey species.

“With fewer sharks around, the species they prey upon – like cownose rays – have increased in numbers, and in turn, hordes of cownose rays dining on bay scallops, have wiped the scallops out,” says co-author Julia Baum of Dalhousie.

“This ecological event is having a large impact on local communities that depend so much on healthy fisheries,” says Charles Peterson, a professor of marine sciences biology and ecology at the Institute of Marine Sciences, University of North Carolina at Chapel Hill and co-leader of the study.

By examining a dozen different research surveys from 1970-2005 along the eastern U.S. coast, the research team has found that their original study underestimated the extent of the declines: scalloped hammerhead and tiger sharks may have declined by more than 97 percent; bull, dusky, and smooth hammerhead sharks by more than 99 percent.

“Large sharks have been functionally eliminated from the east coast of the U.S., meaning that they can no longer perform their ecosystem role as top predators,” says Baum, blaming the shift on growing worldwide demand for shark meat and fins – as many as 73 million sharks are killed each year for the finning trade.

Up until now, researching the consequences of decreasing shark populations has been a challenge.

Ellen Pikitch, a professor at the University of Miami Rosenstiel School of Marine and Atmospheric Science said, “These unforeseen and devastating impacts underscore the need to take a more holistic ecosystem-based approach to fisheries management.”

As shark populations plummeted, their elasmobranch prey-rays, skates, and smaller sharks-rose, according to research surveys looking at the past 16 to 35 years. With an average population increase of about eight percent per year, the east coast cownose rays have thrived the most and may now number as many as 40 million.

The rays eat large quantities of bivalves, including bay scallops, oysters, soft-shell and hard clams, in the bays and estuaries they frequent during summer and migrate through during fall and spring.

“Increased predation by cownose rays also may inhibit recovery of oysters and clams from the effects of overexploitation, disease, habitat destruction, and pollution, which already have depressed these species,” says Peterson, noting shellfish declines in areas occupied by cownose rays and examples of stable or growing shellfish populations in areas beyond the ray’s northernmost limit.

“Solutions include enhancing protection of great sharks by substantially reducing fishing pressure on all of these species and enforcing bans on shark finning both in national waters and on the high seas,” says Baum.

Source: Daily Times