How overfishing can alter an ocean’s entire ecosystem
When you tip the balance, a cascade of other changes may occur.
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“You remove all the fish, and [coral reefs] look like a sewer,” he says. “They’re green and slimy and covered with all this stuff the fish used to eat.”
In the Gulf of Maine urchin experiment, another feedback may have been at work. Without urchins, the ecosystem’s major grazer, seaweed grew thickly, providing more cover for crab populations.
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“We’re left with an oddly stripped ecosystem here in the Gulf of Maine – absent our apex predators and absent our herbivores,” says Robert Steneck, a professor of oceanography at the University of Maine’s Darling Marine Center in Walpole, and Leland’s adviser on the urchin experiments. “We’ve steered this ecosystem to a place for which there is no evolutionary history.”
Scientists value diverse ecosystems for their redundancy. Redundancy – lots of species doing the same thing – equates to more ability to withstand natural or man-made shocks, from an El Niño to global warming. In the tropics, scientists have found that reefs with intact ecosystems recover faster from such disturbances.
They’ve also found that areas off-limits to fishing have greater species richness compared with fished areas, and they experience less fluctuation in fish biomass when disturbed – findings with implications not only for fishermen but also for climate change.
As stocks of bigger fish have grown scarce, fishermen have moved down the food web, chasing invertebrates and small fish. (In Asia, marketers are trying to develop a market for jellyfish, a growing share of their catch.) In parts of eastern Maine where cod and other finfish once ruled, 90 percent of fishermen now rely on lobster. If lobster stocks crash, eastern Maine lobstermen would have nothing to fall back on.
“[Lobsters] are relatives of bugs, and these populations go up and down rapidly,” says Robin Alden, executive director of the Penobscot East Resource Center in Stonington, Maine. “We’ve got an economy here that’s terrifyingly dependent on lobster.”
Ecosystems have proven that they can recover. Since various protective measures were instituted in the Gulf of Maine and Georges Bank in the past decade, some stocks have rebounded. Herring are doing well, as are haddock. But cod, once a mainstay, remain depressed.
Some fault the loss of the big, old females, often the first to be caught. A lone 28-pound red snapper can produce 9.3 million eggs. By one calculation, it would take 212 fish weighing 2-1/2 pounds to produce the same quantity.
Others point to new imbalances: “If you look at the total mass of fish out on Georges [Bank], it’s very stable,” says Steve Murawski, director of the National Oceanic and Atmospheric Association (NOAA) Fisheries Service. “There’s a lot of species, just not the ones you want to put on your white tablecloth.”
Perhaps those newly abundant species are eating young cod. Fishermen up and down the New England coast fault the dogfish, a kind of shark. Scientists say dogfish don’t eat cod, but Jon Grabowski, an ecologist at the Gulf of Maine Resource Institute in Portland, Maine, says that dogfish need not eat cod to influence cod behavior. When dogfish are present, cod tend to disperse, he notes, a phenomenon known as “risk effects” or, more dramatically, the “ecology of fear.”
“One lion is only going to eat one wildebeest every couple of days,” says Grabowski. “But it’s going to induce a herd of wildebeest to move.”