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.
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," says Ellen Pikitch, a professor at the University of Miami Rosenstiel School of Marine and Atmospheric Science and executive director of the Pew Institute for Ocean Science. "These unforeseen and devastating impacts underscore the need to take a more holistic ecosystem-based approach to fisheries management."
As great shark populations plummeted, their elasmobranch prey—rays, skates, and smaller sharks—increased considerably, according to research surveys looking at the past 16 to 35 years. Cownose rays are most conspicuous among the 12 species showing increases because of their near-shore migrations. With an average population increase of about eight 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 four 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 when Peterson sampled bay scallops in North Carolina sounds in late summer before and after the cownose rays passed through, he found that most scallops survived the ray preda
Source:University of Miami Rosenstiel School of Marine & Atmospheric Science