Unintended Consequences
People like to fish, and some of the animals people like to fish for are sharks of all types. Sharks have a reputation as large, ferocious predators, and I’m sure it’s a thrill to land one.
Unfortunately, as with many types of ocean fishes, so many sharks have been killed that their numbers, in some places, are being depleted. In today’s Science magazine, two scientists report that over-fishing of sharks has led to a surprising outcome — depletion of scallop beds along some east coast fisheries.
The connection they found was a surprise because sharks and scallops don’t really interact. So how did shark fishing affect the scallop harvest? One of the scientists had been studying sharks and the other studying scallops. The connection they found lies in the intermediate step in the food chain, cownose rays. Sharks like to eat the rays, and since there are fewer sharks, populations of rays have increased dramatically. And the rays like to eat scallops, which live in shallow water and are relatively undefended and easy prey. So fewer sharks, more rays, and scallops get eaten in larger quantities.
From the Washington Post’s story on this:
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.”
The rays also like to dig in seagrass for clams, and can destroy vital habitat for shrimp and crabs.
Not all scientists are convinced that this research has proven the connection, and further research will no doubt be necessary. But similar ecosystem linkages have long been shown in terrestrial food webs, and this is the first glimpse into similar systemic links among ocean wildlife.
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.”…
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.”
Pictured is a black ray, relative of the cownose ray.