That the earths oceans are in flux is not open to debate. But now a group of researchers suggests that the driving force behind the most detrimental changes to our seas is historical overfishing, as opposed to more recent pressures such as pollution and global warming. "If we want to restore these ecosystems, we have to understand how they function, and just looking back a couple of decades isnt going to tell us," Karen Bjorndal, a zoologist who worked on the study, says. The multidisciplinary report, authored by 19 scientists, appears in today's Science.

The researchers studied a variety of data sources, including modern ecological records, historical information regarding fishing activities from as far back as the 15th century, archaeological records from human coastal settlements dating from 10,000 years ago, and paleoecological data that date from 125,000 years ago. "We started out to study everything that people had ever done to oceans historically and were astounded to discover that in each case we examined, overfishing was the primary driver of ecosystem collapse," says lead author Jeremy Jackson of the Scripps Institution of Oceanography in La Jolla, Calif.

The team also investigated the sea changes in a variety of locales around the world, from the Chesapeake Bay to Caribbean reefs and Australian coastal bays. They found that, around the world, early human hunting of large marine animals near the top of the food chain, such as sharks, turtles or whales, affected animals lower down on the food chain. Animals of similar size that were not initially targeted for food sometimes stepped in to perform the ecosystem roles of the overfished animals. This compensatory action only served to mask the fishings deleterious effect on the worlds oceans, the scientists say.

One synergistic example cited in the paper is the decline of underwater kelp forests in the northern Pacific. As early as 2,500 years ago, sea otters were hunted heavily by humans, which allowed the population of sea urchins, usually kept in check by hungry otters, to swell. The sea urchins, in turn, consumed the kelp and destroyed a crucial habitat for young fishes. Protecting the otters returned a sense of balance to the system, but a perilous one. Subsequent exploitation of the fish population, the food of choice for seals and sea lions, has decreased their numbers. And this decline has forced killer whales to eat otters instead of their preferred prey of seals or sea lions.

Despite the finding that historical human activity may have initiated the collapse of many coastal ecosystems, the scientists suggest that all is not lost. "We noted opportunities for wise management to capitalize on components of the system that are still present and therefore could be conserved and restored," says co-author Charles Peterson, a professor of marine sciences at the University of North Carolina at Chapel Hill.