Planet Earth

Ocean Watch: Cod Killers

The cod, haddock, and other fish off New England's coast, pushed to the brink by overfishing, face a new threat: tiny, tentacled killers that eat fish larvae.

By Kathy A SvitilJun 1, 1998 5:00 AM


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Georges bank, its once teeming schools of cod and haddock nearly exhausted from overfishing, was partially closed to commercial trawlers ten years ago. Fish are still scarce in those New England waters, but one tiny inhabitant of the bank is flourishing. More than a trillion tentacled organisms called hydroids are adrift there. Despite their diminutive size, the hydroids threaten the recovery of Georges Bank. Hydroids feast on the same foods favored by larval cod and haddock. More ominously, hydroids also consume young fish.

Hydroids are not new to the bank, but their abundance—as many as 100 per gallon of water—is unusual. Hydroids have two life stages: the medusa, about two-tenths of an inch long, is a tiny jellyfish and lives suspended in the ocean, eating plankton; the other stage—the hydroid—normally anchors itself to seaweed or rocks on the seafloor, where it waits for eggs, crustaceans, and detritus to drift by. On Georges Bank, however, hydroids now dominate the upper waters.

Researchers aren’t sure how the hydroids left their moorings. Strong currents or storm waves may scour the bottom of the bank and pull hydroids up into the water column. Georges Bank is never deeper than 660 feet. And the hydroids seem concentrated at the center of the bank, which is less than 220 feet deep.

Sketchy historical data do suggest a storm connection. Hydroids were first noted on the bank at the turn of the century. Later, from 1939 to 1941, oceanographers from the Woods Hole Oceanographic Institution conducted the first systematic survey of Georges Bank. In 1940—a stormy year—hydroids abounded. In 1941—a calm year—they were gone.

That’s the extent of the historical record. Over the years the National Marine Fisheries Service has also surveyed Georges Bank. But they always considered hydroids a nuisance—they stick together and get in the way of everything else you are trying to count, like small crustaceans, says Steve Bollens, a biologist at San Francisco State University. So the fisheries service ignored them.

No one ignores them now. Since 1994, researchers with the Global Ocean Ecosystems Dynamics, or globec, program, have monitored hydroid levels. It is not clear yet whether a hydroid-storm connection exists. There haven’t been enough measurements, says Laurence Madin, a Woods Hole oceanographer.

Another possibility is that the hydroids were scraped from the bottom by the heavy chains on commercial trawlers that hold down fishing nets. It’s a hypothesis researchers hesitate to mention, given the strained relations between New England’s commercial fishing industry and the fisheries science community, which called for the bank’s closing.

Whatever caused the hydroid surge, their presence could play a big role in the bank’s future. Young cod and haddock dine primarily on small crustaceans called copepods. So do hydroids. The globec team showed that where hydroids abound, they can consume nearly all the area’s copepods. But hydroids aren’t just competing with the young fish for food. Barbara Sullivan of the University of Rhode Island was the first to find that hydroids consume young cod.

Sullivan made the discovery while examining how the hydroids’ penchant for copepods was affecting the survival of young fish. We thought that hydroids were competitors, but when we put the hydroids in a small container with copepods and larval fish, they ate the fish before they ate any of the prey, she says.

No one knows what impact the hydroids will have on the recovery of the fish stock, or what, if anything, should be done. But it’s important to remember that they are a natural part of the ecosystem. The hydroids are not a new phenomenon, Madin says. What changes their importance is that the numbers of fish larvae are smaller because the numbers of adult spawning fish are lower. So you have the same number of predators you’ve always had, and fewer prey. The odds are against the prey.

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