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How overfishing can alter an ocean’s entire ecosystem

When you tip the balance, a cascade of other changes may occur.

Cod and haddock are unloaded and sorted at the pier of Lydia's Cove in Chatham, Mass. Cod are brown in coloring and haddock are gray with a black stripe. The depletion of fisheries is affecting the future of the fishing industry on Cape Cod. Fishermen are working alongside scientists and community organizers to adapt their ways in hopes of establishing sustainable ways of fishing – and maintining fishing communities.

Mary Knox Merrill / The Christian Science Monitor

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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.

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