Unusual fish threat afloat in the Atlantic.

Heavily stocked with cod, haddock, and other cold-water fish, Georges Bank in the Northwest Atlantic Ocean was for centuries a magnet for fishermen. Now closed because of overfishing, the steep underwater mountain pulls in researchers studying this important ecosystem's recovery and its vulnerability to climate change.

In 1994, an unusual phenomenon resurfaced from that work. Clear, gelatinous creatures that normally live as tiny, tentacled stalks on the ocean floor were filling up the nets used to sample the waters. "We noticed nets were coming up slimy," recalls Barbara K. Sullivan of the University of Rhode Island Graduate School of Oceanography in Narragansett.

An obscure report from 1915 also described large numbers of the organisms, which turned out to be hydroids, cousins of jellyfish and anemones.

From their studies of the hydroids at Georges Bank and in the lab, reported at several forums this fall, Sullivan and her colleagues are amassing evidence that the floating creatures (Clytia gracilis) are unrecognized, yet major players in the Georges Bank food web. Hydroids may be competing with cod and haddock larvae by consuming significant amounts of minute crustaceans known as copepods, which the fish also eat.

The hydroids, with their stinging tentacles, prey directly on fish larvae, too. Sullivan reports that in large tanks, hydroids at the densities found in Georges Bank can reduce the survival of fish larvae by 50 percent.

The floating forms of hydroids may account for 90 percent of the planktonic biomass pulled up in sampling nets, says Laurence P. Madin of the Woods Hole (Mass.) Oceanographic Institution, one of Sullivan's collaborators. "Normally these things wouldn't be found floating in the water at all," he adds.

"Biologically, it's interesting because it's something you don't suspect is going on," says Peter Auster, a fish ecologist at the University of Connecticut in Groton. Researchers point to evidence that the ocean turbulence around Georges Bank is a big factor in keeping the hydroids afloat.

They've also floated another possibility: Commercial trawling for fish, which scours the seafloor habitat (SN: 10/26/96, p. 268), may have stirred up the hydroids. "Most of Georges Bank prior to the recent closing was trawled extensively," says Madin. "It seems like a reasonable hypothesis, although it may only be part of what's happening."