Other Environmental Themes

Shark Losses Cut Deep

By design, fishing targets animals at the top of the food chain. Using this narrow criterion, the commercial fishing enterprise of the last half century has been a tremendous "success." Inventories of large, predatory species, such as tuna and swordfish, have dropped throughout the world's oceans more than 90 percent since the introduction of modern industrialized fishing techniques in the early-1950s. A single bluefin tuna, which can weigh more than 1,000 pounds, often fetches more than $10,000 in today’s market, and that financial incentive has contributed to the fish’s demise.

Few species sit higher on that chain than sharks, and none has suffered more at the hands of man. Despite their terror-inducing reputation, sharks kill only about 10 people each year. (Jaws author Peter Benchley once quipped that more people die each year falling off their toilets than are killed by sharks.) Humans, on the other hand, kill about 100 million sharks each year—more than half of them caught accidentally by fishermen seeking other quarry and discarded as "bycatch."

A 2003 study in Science determined that shark populations in the North Atlantic declined more than 75% between 1986 and 2000. Hammerhead populations declined almost 90 percent. "Sharks are in a global extinction crisis," claims one of the study's authors, Ransom Myers of Dalhousie University in Halifax, Canada. Myers and his Dalhousie colleague Julia Baum found that the once common oceanic whitetip sharks has almost completely disappeared from the Gulf of Mexico, falling to one percent of their 1950's levels. Researchers expect it will be difficult for sharks to rebound, given their slow growth rates and long reproductive cycles.

Beyond the extinction threat, shark losses exact a broader ecological toll. Enric Sala of the Scripps Institution of Oceanography and his collaborators concluded that sharks play a vital role in maintaining the health of coral reefs. Removing these predators through overfishing sets off a cascade of effects that ripple through the food chain. A decrease in the Caribbean shark population is met by an increase in its prey, the grouper fish. The expanding grouper population takes parrotfish, normally responsible for clearing coral of algae, in greater numbers. This could explain why algae now dominates many degraded reefs in the Caribbean. It also shows how the systematic elimination of one species—a key link in a complex web of relationships—can destabilize the entire ecosystem.