Overfishing of powerful sharks — a top predator in the ocean — may endanger bay scallops, a gourmet delicacy.
With fewer sharks to devour them, skates and rays have increased sharply along the East Coast and they are gobbling up shellfish, particularly bay scallops, researchers report in Friday’s issue of the journal Science.
Ecologists have known that reducing key species on land can affect an entire ecosystem, but this study provides hard data for the same thing in the ocean, said lead author Charles Peterson of the Institute of Marine Sciences at the University of North Carolina.
Co-author Ransom Myers of Dalhousie University in Halifax, Nova Scotia, and Peterson were studying different ends of the food chain, Peterson said in a telephone interview.
“Myers was working on great sharks and I was working on cownose rays and their impact on bay scallops and other shellfish. We realized that separately we had interesting science, but together we had an absolute revelation,” he said.
“We were able to show why these top predators matter,” Peterson said. “We knew the answer right there, that there was a consequence.”
Peterson, who works at the university center in Morehead City, N.C., said scallops used to be so abundant there that people were allowed to collect a bushel a day. “The kids were able to see that food doesn’t just come from a market,” he said.
Now, scallops are very reduced, he said. “The rays, as they come through, eat all that are in any dense patch and have eaten so many there does not appear to be enough to create spawning stock.”
Scallops are an easy target because they do not burrow into the sand, Peterson said. Millions of rays from Chesapeake Bay migrate through the area, he said. “What are they going to feed on to fuel their migration?”
In some areas they enter seagrass beds and dig up clams, but that is an important nursery habitat for shrimp, blue crabs and fish, Peterson said, “so there is a high concern that we may now be cascading to habitat destruction.”
Expert: Link still 'tenuous'
Not so sure was Steve Murawski, director of scientific programs at the National Marine Fisheries Service. He said the links between the large sharks, medium size rays and bay scallops were “tenuous.”
There is very little food and feeding data on the rays, he said, and in terms of the decline of bay scallops, habitat degradation and environmental issues could be factors, too. As for the increase in rays, he said, they used to be widely caught and discarded and fishing has declined in their prime habitat.
Murawski, who was not part of the research team, said he is not saying there is no relationship among the sharks, rays and scallops, only that other factors also need to be considered.
Robert E. Hueter, director of shark research at Mote Marine Laboratory in Sarasota, Fla., said scientists have warned about the effects of shark depletion for years but there have been few studies to back them up.
This report, he said, “demonstrates plausible links between the decline of sharks, the subsequent rise of their prey, and the resulting decline of those prey species’ prey. You don’t have to be a marine biologist to grasp this connection.”
“Scientists will now debate the specific numbers and correlations in this paper, and sadly, Dr. Myers will not be around for that debate,” he said. Myers, 54, died Tuesday in Halifax.
Keeping the balance
Hueter, who was not part of the research team, said the “overall message is important and true: If we take out whole segments of ecosystems, especially top predators like sharks, the balance among species is toppled, and the effects cascade throughout the system. And some of those effects — such as a negative impact on other important fisheries, as in the Myers study — can be long-term and deleterious to human society.”
Ellen Pikitch, executive director of the Pew Institute for Ocean Science, said scientists have known that the loss of the great sharks would ripple through the ecosystem in some way, but this is “the first study to show consequences with hard data.”
Pikitch was not part of the research team, but the Pew Institute helped support the work.
Other funding came from the Sloan Census of Marine Life, Natural Sciences and Engineering Research Council of Canada, Killam Trust, North Carolina Fisheries Resource Grants Program, North Carolina Sea Grant and the National Science Foundation.