Overfishing of one of the ocean's top predators has triggered a lethal chain reaction that threatens to decimate populations of bay scallops, oysters, and clams, warns a new study. Plunging populations of 11 large shark species have caused a spike in the numbers of their prey, including smaller sharks, rays, and skates. These creatures in turn are wiping out other marine organisms and may be destroying sea grass habitat, which serves as a nursery for many species.
Sharks have had a rough few decades. Demand for their fins and meat has resulted in increasing exploitation, and the creatures are often accidentally captured by swordfish and tuna fishers. As many as 73 million sharks are killed annually, and past studies have indicated many populations have been cut by half. Nonetheless, scientists have had difficulty gauging the decline in several shark species over the years because of a lack of data. The effect of this decline on ocean ecosystems has also proven hard to measure.
To get a better grip on the problem, marine biologist Julia Baum of Dalhousie University in Halifax, Canada, and colleagues trawled 35 years of marine surveys and fisheries data to document declines in large sharks off the eastern seaboard of the U.S. The numbers, reported today in Science, are stark: The blacktip shark population has plummeted 93% since 1972, with similar declines for tiger, bull, and smooth hammerhead sharks.
Simultaneously, the abundance of 12 of the 14 species that these sharks prey on--including the little skate, the chain catshark, and the cownose ray--increased dramatically. The latter has seen its numbers swell ten-fold. The jump in ray numbers is especially troubling, notes co-author and marine biologist Charles Peterson of the University of North Carolina at Chapel Hill, because the fish shred the sea grass that houses crabs and clams. In fact, he notes, by 2004 ray predation had shut down North Carolina's century-old bay scallop fishery.
The 11 other shark prey species whose populations rose over the 35-year period could be having similar impacts to that of the cownose ray, says Baum.
"The result is profound" because the high diversity of fishes in the northwest Atlantic has been thought to minimize catastrophic changes in the food web, says marine scientist Kenneth Frank of the Bedford Institute of Oceanography in Dartmouth, Canada. Sharks are only part of the puzzle, however, Frank warns. People's appetites for oysters, clams, and scallops are likely having a more direct impact on these populations, he says.