Science / Medicine : 2 Exotic Lake Visitors Have Officials Worried

Discovery of a new species of fish and plankton in Lake Superior has natural resources officials worried.

Herb Johnson, Duluth-area fisheries manager for the Minnesota Department of Natural Resources, said the river ruffe, a perch-type fish, and a plankton called a cladoceran, a predatory link in the food chain, probably got into the lake from Northern Europe by hitching a ride in the fresh-water ballast tanks of an oceangoing ship.

Johnson said the Wisconsin Department of Natural Resources discovered the fish in the St. Louis River last summer and corroborated it this spring.

“It’s very similar to our yellow perch. It occupies similar habitat and has similar eating habits,” said Johnson. “Like any exotic, it is a threat to our native species. It is not necessarily rapacious but it will compete for habitat and food with our native species.”

The ruffe’s native range is from Siberia to England, Johnson said. The fish, which probably arrived in the United States in egg form, is now living on the southern edge of its normal range and so the department anticipates better-than-average growth. The fish, which normally reaches 8 to 10 inches in length, has been living in the river and lake for at least two years.

“Like our perch over here, they are considered by some to be good eating and by others to be a pest,” said Johnson, who wants anglers to be on the lookout for specimens.

Johnson said the almost microscopic plankton is “probably more scary” than the ruffe because it is a predator and the department has no idea how to eradicate or even contain it. The plankton was first spotted in southern Lake Huron in December, 1984, and made its first appearance in Lake Superior last summer.

Another disturbing trait, he said, is that the plankton’s eggs can survive being frozen or dried out, which means if they adhere to a net or other fishing gear they can be transported unnoticed from one lake to another, even the following year.