A team of Canadian and American ecologists, led by world-renowned fisheries biologist Ransom Myers of Dalhousie University in Halifax, Nova Scotia, has found that overfishing in the Atlantic of the largest predatory sharks, such as the bull, great white, dusky and hammerhead sharks, 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," said co-author Julia Baum of Dalhousie.
"This ecological event is having a large impact on local communities that depend so much on healthy fisheries," said Charles Peterson, a professor of marine sciences biology and ecology at the Institute of Marine Sciences at the University of North Carolina at Chapel Hill and co-leader of the study.
In 2003, Myers and Baum published a study in Science that showed rapid declines in the great sharks of the northwest Atlantic since the mid-1980s. In the new study, funded by the Pew Institute for Ocean Science, the research team examined a dozen different research surveys from 1970-2005 along the eastern U.S. coast and found that their original study underestimated 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.
"The extent of the declines shouldn’t be a surprise considering how heavily large sharks have been fished in recent decades to meet the growing worldwide demand for shark fins and meat," Baum said.
Sharks are targeted in numerous fisheries, and they also are snagged as bycatch in fisheries targeting tunas and swordfish in both U.S. and high-seas fisheries. As many as 73 million sharks are killed worldwide each year for the finning trade, and the number is escalating rapidly.
With an average population increase of about 8 percent per year, the East Coast cownose ray population may now number as many as 40 million. The rays, which can grow to be more than 4 feet across, 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.
In the early 1980s Peterson sampled bay scallops in North Carolina sounds in late summer before and after the cownose rays passed through and found that most scallops survived the ray predation, allowing the scallop population to support a fishery and still replenish itself each year. In contrast, sampling in recent years by Peterson and co-author Sean Powers of the University of South Alabama and the Dauphin Island Sea Lab – after the cownose ray population explosion – showed that the migrating rays consumed nearly all adult bay scallops in the area, except those protected inside fences that the researchers had put up to keep the rays out. By 2004, cownose rays had completely devastated the scallop population, terminating North Carolina’s century-old bay scallop fishery.
"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," said 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.
Ecologists have long predicted that the demise of top predators could trigger destructive consequences. Researching such effects, however, has been a challenge.
"This is the first published field experiment to demonstrate that the loss of sharks is cascading through ocean ecosystems and inflicting collateral damage on food fisheries such as scallops," said Ellen Pikitch, executive director of the Pew Institute for Ocean Science and a professor at the University of Miami Rosenstiel School of Marine and Atmospheric Science. "These unforeseen and devastating impacts underscore the need to take a more holistic ecosystem-based approach to fisheries management."
"Maintaining the populations of top predators is critical for sustaining healthy oceanic ecosystems," said Peterson. "Despite the vastness of the oceans, its organisms are interconnected, meaning that changes at one level have implications several steps removed. Through our work, the ocean is not so unfathomable, and we know better now why sharks matter."
Solutions to the problem, Baum said, "include enhancing protection of great sharks by substantially reducing fishing pressure on all of the shark species and enforcing bans on shark finning both in national waters and on the high seas."
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