Rare Predators Threatening Already Vulnerable East Coast Fish

November 12, 1997

(Editors: a compressed image of the hydroids is available on SFSU's web site a http://www.sfsu.edu/~pubaff/prsrelea/fy97/040.htm. To download, click on image.)

SAN FRANCISCO -- Tiny, tentacled sea creatures, rarely seen drifting in the ocean, have been discovered thriving by the millions off New England's vulnerable Georges Bank over the past few years, threatening valuable cod and haddock, species that have already been decimated by overfishing in the area.

Normally rooted to the sea bottom, the voracious animals, called hydroids, have been found floating free in concentrations as high as 100 per gallon of water, said Steve Bollens, associate professor of biology at San Francisco State University and a leader of the team studying the predatory animals. Each about a tenth of an inch across, they eat most of the daily production of small crustaceans, or copepods, which the fish larvae rely on. As a result, the hydroids have become one of the chief competitors of the commercial fish and may threaten their survival, Bollens said.

Adding insult to injury, the hydroids also appear to kill the young fish directly. "We've seen them with their tentacles engulfing the head of larval cods," Bollens said. "When you add their potential impact as a predator of the fish, you can't ignore their importance."

Bollens, who also holds a position at SFSU's Romberg Tiburon Center for Environmental Studies, presented the latest understanding of the Georges Bank hydroids at a seminar Wednesday (Nov. 12) at the Southwest Fisheries Center of the National Marine Fisheries Service in Tiburon, California. He is part of a team of marine scientists from seven institutions in Canada and the U.S. studying the hydroids' impact on the already-threatened Georges Bank cod and haddock populations. Other team leaders are Laurence Madin, chairman of the biology department at Woods Hole Oceanographic Institution, and Barbara Sullivan, professor of oceanography at University of Rhode Island.

Hydroids are related to jellyfish and have two major life stages, one sedentary and one drifting. But although the predatory Georges Bank hydroids are drifting through the waters, they are clearly in their normally sedentary life stage. Some still trail tendrils that normally secure them to the ocean floor. Why and how they have become free-floating, uprooted armies is a major part of the group's research focus.

Until now, no one has examined this behavior and its impact on local fish populations. "When we first recognized how many were in the waters, we asked each other how could this not have been studied before," Bollens said. "We wonder if this kind of predation has a significant impact in other areas."

The new research is part of the five-year, $25-million Global Ocean Ecosystem Dynamics project, or GLOBEC, funded by the National Science Foundation and the National Oceanic and Atmospheric Administration to study the Georges Bank and its threatened fishery.

The hydroids may have been ripped from their moorings by seasonal storms, or, more ominously, by commercial trawlers, Bollens said. Every square foot of the massive Georges Bank region has been scraped by the heavy chains used to hold down trawling nets and stir up fish. The chains are known to disturb the habitat, and they may account for a sizable number of the dislocated predators that plague the fish and their prime food source.

A huge, traditionally rich fishing region off Canada and New England, the Goerges Bank has for generations been one of the world's most productive fishing waters. But in the past 10 years, fishing has declined so precipitously that much of the area has been closed off, in hopes the fishery will recover .

The Romberg Tiburon Center for Environmental Studies is the marine sciences research arm of San Francisco State University, the second largest of the nationally recognized 23-campus California State University System.


Wallace Ravven
Senior Science Writer
San Francisco State University
SF, California, USA 94132

San Francisco State University

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