Great Lakes fish food in decline

CHICAGO -- A quarter-inch crustacean that makes up the bulk of the diet for some Great Lakes fish is disappearing from some areas, and researchers blame the invasion of non-native zebra mussels.

The spineless scud, also called diporeia, is a shrimplike bottom dweller that makes up 25 to 75 percent of the whitefish diet, according to the National Oceanic and Atmospheric Administration. It also feeds sculpin, smelt and chub, which in turn feed trout and salmon.

Tom Nalepa, research biologist with the Great Lakes Environmental Research Lab, said Lake Michigan scud declined 68 percent from 1994 to 2000. They still are found along the Wisconsin shore but have vanished from Chicago, Gary, Ind., and the Michigan shore.

Nalepa wrote about the decline in "The State of the Great Lakes 2003," a report released last week as a joint project of the U.S. Environmental Protection Agency and Environment Canada. The report also notes a scud decline in Lake Ontario.

"The decline is so dramatic that we're worried about what it will do to the food web and the upper-level fisheries," Nalepa said.

Researchers believe zebra mussels, transported to the lakes in ship ballast water in the 1980s, are starving scuds by straining the algae they eat out of the water.

But Nalepa said scientists also are looking into the possibility of disease as a cause of the decline.

"It just doesn't make sense that an animal that is not infected is dying off completely," he said.

Biologists have noticed changing feeding habits and thinner whitefish with the decline. Zebra mussels found in whitefish stomachs indicate a switch to the less nutritious mollusks, which are harder to digest.