Portable Planet

A team of Canadian and American scientists says that the overfishing of large sharks has led to an explosion of small predators that are devastating populations of shellfish. Sharks usually do not evoke sympathy, but a new study could change that.

Researchers from Dalhousie University in Nova Scotia and the Universities of North Carolina and South Alabama find that the dramatic declines in 11 species of large sharks has cascaded down through the ocean food chain with a negative impact on commercial shellfisheries.

“What this study tells us is that sharks matter and by implication other top predators of the sea matter,” explained Charles Peterson of the University of North Carolina. “The magnitude of those declines has consequences on the rest of the food web.”

By analyzing several independent research surveys and fisheries records, Peterson and his colleagues report in the journal Science that the steep drop in shark numbers along the U.S. Atlantic coast since 1970 has caused an increase in 12 of 13 species they studied of lesser predators - rays, skates, and smaller sharks eaten mainly by large sharks.

The abundance of one of these species, the cownose ray, has increased 20 times and its prey - scallops, clams, oysters, and other shellfish - have been reduced to the point that commercial shellfisheries along the U.S. East Coast have suffered.

In an interview with Science magazine, Peterson describes the rays’ impact on bay scallops along the coast since the mid 1980s, when surveys showed the scallop population to have been stable.

“The rays were so abundant that they essentially eliminated all of the bay scallops except for ones that we protected by building a stockade of vertical posts spaced so that the rays could not get between them,” he added.

Peterson says exploitation of large sharks has intensified worldwide in recent decades, because of rising demand for their meat and fins. But they have also been caught inadvertently.

“That’s because fishing with its gear - in this case, it’s long line gear - tends to be very non-selective within the group that is being fished,” he explained. “Here, it is the group that will eat bait on a large hook, and that affects all of these large shark species.”

The scientists say that this is the first study to show what effect the long-known shark decline has had on the ocean ecosystem.

It is part of a broader global decimation of sea creatures noted in a November paper in Science by another team of North American marine biologists. They found that one-third of fish species have collapsed so far, meaning that their catch has declined 90 percent below the historic maximum. The lead researcher of that study, Canadian Boris Worm of Dalhousie University, says seven percent of them have become extinct.

“If this trend continues, if we don’t change the way we are managing ocean ecosystems, this trend projects that 100 percent of species will collapse by the year 2048 or around that,” he noted.

Source: VOA