NOAA 2001-R505
Contact: Jana Goldman


Whitefish in the northern part of Lake Michigan hoping to find some of their favorite snack – the tiny shrimp-like Diporeia - might be out of luck, say scientists from the Department of Commerce's National Oceanic and Atmospheric Administration.

"We are seeing the numbers of this important food source decline in the northern part of Lake Michigan," said Thomas Nalepa, a research biologist at NOAA's Great Lakes Environmental Research Laboratory in Ann Arbor, Mich., which studies Great Lakes issues. "We knew they were declining in the southern part of the lake, but did not expect to see such a dramatic decline in the north."

Nalepa will present his findings at the International Association for Great Lakes Research meeting June 11 and 12 in Green Bay, Wis.

In 2000, Nalepa took samples from 185 monitoring stations in Lake Michigan.

"We found large areas were there were no Diporeia at all," he said.

Scientists are not exactly sure what is causing the decline, but note that it coincides with the arrival and increase of the zebra mussel, an invasive species. The zebra mussel could be competing with the Diporeia for food. A tiny shrimp-like organism, Diporeia lives on the mud of the lake bottom and eats the settling algae from the water column. NOAA scientists believe that the Diporeia loss is because of the competition for algae with the zebra mussel.

"Over the entire lake, the Diporeia population declined by 68 percent between 1994 and 2000," Nalepa said.

The decline of Diporeia has affected the feeding patterns of many of the lake fish, especially whitefish. Commercial fishers of whitefish report skinnier fish. Some fish were so thin that it was impossible to get a fillet from them. Biologists have found zebra mussel shells in the stomachs of whitefish.

"The fish are eating the zebra mussels which are not as nutritious for them, plus the shells are not digestible," Nalepa said. "Diporeia is a high-calorie, high-fat food. Zebra mussels are not."

Diporeia made up 25-75 percent of the whitefish diet contributing to as much as 61 percent of its weight.

Whitefish is not the only fish affected. Sports fish such as sculpin, smelt, and chub, which are prey for trout and salmon are also being affected. These smaller prey fish feed heavily on Diporeia and their feeding patterns, numbers, and distributions are changing because of the loss of the small crustacean.

There are five Great Lakes: Huron, Michigan, Ontario, Erie, and Superior. Nalepa said that the Diporeia decline is spreading into Lake Huron, and has been seen in parts of lakes Erie and Ontario. Samples taken in Lake Huron in 2000 show that Diporeia is now almost completely gone from the southern portion of that lake.

Among his materials, Nalepa has charts of Lake Michigan showing Diporea density. Every few years, the areas of dark blues, which indicate highest densities, grow lighter and lighter in color.

"The decline first happens close to shore. Then it moves offshore to deeper water," Nalepa explained." It is difficult to predict what will eventually happen in these deep areas. In 10 to 20 years, these areas may be completely devoid of Diporeia, or they may be the only places where Diporeia is found."

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