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New food chain threat musseling into Laurentian Great Lakes
Thursday, February 27, 2003

Muskegon Chronicle, Michigan By Dave LeMieux

It's a food-chain fight, and the fish we love to catch are losing.

Scientists believe a cousin of the pesky zebra mussel, the quagga mussel, is in the midst of a population explosion that endangers the food chain in Lake Michigan and other Great Lakes. The quagga is larger and can live in deeper water than the zebra, adding to the potential havoc.

The quaggas' prolific breeding is a bad sign for the lakes. They may soon be the dominant foreign invader mussel in the lakes.

Researchers have always suspected there's a link between zebra mussels and the declining health of Lake Michigan's whitefish population. As the mussels proliferate, they take food away from creatures who themselves are important lake menu items.

Both zebras and quaggas filter-feed tiny food particles. So does diporeia, a tiny shrimplike creature that is a favored food of many Great Lakes fish, including the whitefish.

So far, the diporeia appear to be losing. Tom Nalepa, a research biologist at NOAA's Great Lakes Environmental Research Lab in Ann Arbor, is part of a research team trying to solve the mystery of the disappearing diporeia.

He said diporeia have largely vanished from large areas in Lake Michigan. Nalepa suspects it's because zebra mussels are intercepting diporeia's food; the areas where diporeia have disappeared roughly coincide with the shallow waters where zebra mussels have proliferated.

But what's confounding scientists is the fact the diporeia are also vanishing in the shallow waters where there aren't any zebra mussels. "It's maddening, not knowing the cause of the decline," said Nalepa. "It seems to be food, but the exact mechanism is unclear."

Zebra mussels like shallow water and are rarely found in water deeper than 150 feet. Quagga mussels can live in shallow water as well as deep, cold waters. That means diporeia's competitors are spreading in all lake depths.

When feeding, zebra and quagga mussels suck in a lot more food than they can eat and spit out what they don't need. Diporeia, not surprisingly, won't eat what the mussels have spit out. And there's not much left in the water to eat once the mussels are finished. With their preferred food diporeia disappearing, whitefish have switched to eating zebra mussels.

Zebra mussels are harder to get at and aren't nearly as nutritious as diporeia were, so the whitefish are maturing more slowly and getting skinnier and skinnier, said Nalepa. Nalepa said the same scenario could be extended into the lake's deep waters by the quagga mussel. In that case, the quagga mussels would be robbing a deep-water fish called sculpin of their food. Lake trout feed on sculpin, so reduced sculpin populations would in turn likely mean a decrease in the numbers of lake trout.

Three years ago, zebra mussels outnumbered the slightly larger quagga mussels 20 to 1. The ratio now is closer to 3 to 1, say scientists at the National Oceanic and Atmospheric Administration. It's likely the faster-reproducing quagga mussels will soon vastly outnumber zebra mussels in Lake Michigan, as they do already in Lake Erie and Lake Ontario. Quagga mussels now outnumber zebra mussels 10 to 1 in some spots in western Lake Erie.

Adult quagga mussels are about 1 1/2 inches long. Zebra mussels grow to just under 1 inch.

For now, more quagga mussels doesn't mean there will be fewer zebra mussels.

"It's not as if the zebra mussel population will stop growing, it's just that the quagga mussel population will expand more rapidly than zebra mussels," said Nalepa. Eventually, zebra mussel numbers could start falling, he said.


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