As sharks vanish, chaotic new order emerges

Major declines in large sharks along the US coast have in turn triggered declines in shellfish and reduced water quality, proof that the ocean's food web is collapsing, a groundbreaking new study reveals. With the virtual elimination of large sharks along the US east coast, such as black tip and tiger sharks, the species they used to eat–small sharks, rays and skates–have boomed in numbers. Cownose ray populations increased 20-fold since 1970 and as a direct consequence, shellfish like scallops that the cownose ray eats have been nearly wiped out despite major conservation efforts. The cascade of impacts resulting from over-fishing large sharks goes further still, according to marine scientist Ransom Myers, who co-authored a document in a paper published on Mar. 29 in Science. The loss of scallops has reduced water quality because scallops and other shellfish filter sea water. And the cownose ray is now feeding voraciously on other shellfish, like oysters and clams. "We've also seen large seagrass beds the rays have dug up looking for shellfish," says co-author Charles Peterson of the Institute of Marine Sciences at the University of North Carolina. Seagrass is considered the "nursery" for many fish species, but Peterson says there is no data available on what the impacts the rays are having on seagrass beds. The cownose ray is just one of 12 species of rays, skates and small sharks that have sharply increased in numbers. "We have no idea what impacts the other 11 are having," Peterson said. "What we do know from this study is that sharks play a crucial role in the ocean ecosystem." Sharks are the ocean's "king of the beasts," the top or apex predator. Ecologists have learned that top predators perform an important role of stabilizing ecosystems. Their removal produces what's termed a "trophic cascade." For example, sharp declines of sea otters on the North American west coast resulted in a boom of the otter's favorite food, sea urchins, which in turn led to major declines in kelp forests. Kelp forests are another kind of important nursery for many marine species. Going the other way, reintroducing wolves, the top predator, into Yellowstone Park reduced populations of elk and moose. That led to a resurgence in plants and tree growth along streams and rivers, benefiting birds and other species, according to Jim Estes, a marine ecologist at the University of California. "Wolves may help buffer ecosystems from climate change, according to some recent research," Estes said in an interview last year. "On land, many of top predators are gone but the oceans still have all the pieces if we stop exploiting them." Those pieces, and especially the top predators, are now going fast. In 2003, Ransom Myers and Julia Baum of Canada's Dalhousie University documented that more than 90 percent of all large predatory fish in the world's oceans were gone, mainly due to over-fishing. They warned that this loss of the entire top of the ocean food chain would unbalance ocean ecosystems. "We have literally chopped off the top of the ocean food web," Baum said. "As a result, there is a massive restructuring [of the food web] going on." This current study is perhaps the first ever documentation of the cascading impacts of the loss of large sharks because ocean food webs are so complex and there is little data. "There is little doubt this is kind of thing is happening in other coastal regions of the world," Baum said. She noted that Japanese scientists have found that large increases in eagle ray populations are responsible for wiping out their natural and aquaculture shellfish beds, but are puzzled as to why. "It should not be a surprise given the massive decline in sharks that feed on rays," she said. Shark finning has become the major reason for the decline. Fishers net sharks, slice off the fins, then throw the bleeding, limbless fish back into the water. Demand is growing at more than five percent a year and efforts to ban the practice have not been effective, she added. The United States and Canada placed bans on finning in the early 1990s, while 60 other countries have agreed to ban the practice in the Atlantic Ocean. Mexico announced a ban this month. But these bans are not only hard to enforce, they often are not really bans at all, says Baum. Instead, finning is deemed acceptable as long the body of the shark is taken as well. The escalating demand for shark fins and cartilage is producing a catastrophic decline in the numbers of sharks around the world. One major step is to reduce public demand for shark products through education. The other is to have a complete and absolute ban on catching large coastal sharks, says Peterson. "The results of our study should be taken very seriously," he said.