How overfishing can alter an ocean’s entire ecosystem

In 2000, University of Maine graduate student Amanda Leland began a seemingly straightforward restoration project. She transported 24,000 young sea urchins, which are native to the Gulf of Maine, to an area where overharvesting had caused them to disappear. She expected to watch them thrive and repopulate. But something else happened: An army of Jonah crabs arrived and, within a month, the hand-sized predators had devoured the urchins.

Ms. Leland repeated the experiment the following year. But this time she transplanted the urchins in spring, months before the crabs’ fall migration. They thrived as expected – until August when the crabs showed up. By Sept. 1, they were gone. Leland thought she knew why. With cod and other groundfish gone, Jonah crabs were four times more abundant than in times past.

“There really aren’t many crab predators left,” says Leland, now the Environ­ment­­al Defense Fund’s national policy director of oceans in Washington. “They have been released from predation control.”

Scientists have documented versions of this story around the world. Overfishing has shifted entire ecosystems with often surprising, and occasionally unpleasant, results. In the tropics, seaweed often dominates where coral once reigned. Around the world, jellyfish and algae proliferate where finfish previously dominated. With big predators often gone or greatly depleted, organisms lower on the food web grow more abundant, reducing their own prey in turn.

Some say this is worrisome evidence of a greatly changed and simplified marine ecosystem. Like investment portfolios with few holdings, simple ecosystems are prone to collapse; and collapsed or rearranged ecosystems don’t necessarily provide what humans expect. Increasingly mindful of marine ecosystems’ complexity – and wary of their collapse – some people are calling for a holistic approach to managing ecosystems, one that aims to manage for the health of the entire system rather than that of a single stock.

Just 4 percent of the world’s oceans remains free from human impact, according to a 2008 study in the journal Science. Forty percent of this is heavily impacted.

Where intact ecosystems remain, scientists are often astounded by what they find. On the remote Palmyra Atoll in the equatorial Pacific, for example, large sharks and predatory fish dominate the reefscape – an “abundance of toothy things,” says Callum Roberts, a professor of marine conservation at the University of York, England. Unlike terrestrial ecosystems, which are dominated by a few apex predators, pristine marine ecosystems support a large biomass at the top.

“Today’s oceans have got far less in the way of biomass than they used to,” Professor Roberts says. “We’re altering ecosystems in a way that reduces the level of productivity they can support.”

By one estimate, only one-tenth of the sharks, tunas, cods, and other large predatory fish that once swam the oceans remains. And their absence has ripple effects throughout marine food webs.

In the eastern US, one study found that the loss of large predators (sharks) let medium-sized predators (skates) in­­crease in bays and estuaries. They, in turn, decimated the bay scallop fishery.

In tropical reefs, scientists think that fishing has removed fish that eat starfish. Starfish graze on coral. Eighty percent of Caribbean reefs have disappeared in the past 30 years. (Reefs in the Pacific are faring slightly better.)

Around the world, loss of fish, combined with increased nutrient inflow from pollution, has caused a bloom of primitive organisms in the ocean: the same algae, bacteria, and jellyfish that dominated the seas before the explosion of complex life 600 million years ago. Jeremy Jackson, a professor of oceanography at Scripps Institution of Oceanography in La Jolla, Calif., has dubbed it “the rise of slime.”

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