Invasive Species

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Rainbow Smelt (Osmerus mordax)

Photo by Department of Game, Fish, and Parks, South Dakota

Photo by Department of Game, Fish, and Parks, South Dakota

DESCRIPTION: Rainbow smelt is a very small (7 to 9 inches long, weighing 3 ounces) invasive fish. They are silvery overall with a pale green back and iridescent purple, blue and pink on their sides. They have a conspicuous silvery streak running lengthwise along each side. In the water, rainbow smelt shimmer colorfully. When removed from the water, however, they quickly fade to a silvery white and give off the odor of cucumbers.

DISTRIBUTION AND HABITAT: Once exclusively an anadromous species (residing in saltwater but entering freshwater to reproduce) of the east and west coasts of North America, the smelt has been successfully introduced into freshwater systems throughout the northeastern and central United States. Introduced to Michigan's Crystal Lake as food for stocked Atlantic salmon in 1912, this small ocean fish soon escaped to Lake Michigan. By 1930, the rapidly growing smelt population had expanded into Lake Superior, and is now found in Lakes Huron, Ontario and Erie as well. Smelt have been present in Wisconsin waters of the Great Lakes for over 70 years, first discovered in 1928 in Little Sturgeon Bay in Door County. Through the intentional or accidental efforts of private individuals, smelt began to spread to Wisconsin’s inland waters in the 1980’s.

In the Great Lakes, rainbow smelt are found in both coastal and offshore habitats. Since they are light-sensitive, they prefer deeper, cooler waters during the warmer seasons, but will favor shallower coastal areas for feeding as winter approaches.

LIFE HISTORY AND EFFECTS OF INVASION: Few smelt live beyond 5 years of age. Both sexes become sexually mature at about 2 years of age. A female may produce 33,000 to 75,000 eggs in one season, depending on her size. The spawning season takes place in early spring. Larvae and juveniles feed upon zooplankton, particularly microscopic crustaceans. Adult smelt feed on small crustaceans and fish.

Although their impact on Great Lakes ecosystems has not been well-documented, fish biologists have clear data showing distinct negative impacts of smelt on the sportfish populations of Wisconsin’s inland lakes. For example, smelt compete directly with juvenile walleye for food, which may be a principal mechanism in limiting walleye recruitment. They compete with other native fish for food as well. Smelt will even eat other fish in their early or larval life stages. These larval fish or fry can include lake trout, whitefish, walleye and cisco, a forage species native to Wisconsin.

There is some evidence that the rate of mercury accumulation in top level fish consumers accelerates when they switch to a diet of smelt. Rainbow smelt are also rich in thiaminase, an enzyme that destroys thiamin, which is necessary for development of fish embryos; as lake trout and other sport fish consume the smelt, their ability to successfully reproduce diminishes because of thiamin-deficiency.

Rainbow smelt has come to represent an important component of the recreational and commercial fishery. In 2004, commercial trawlers in Wisconsin harvested 155,000 pounds of rainbow smelt from Lake Michigan and Green Bay; the peak harvest in recent years was 1,800,000 pounds (1990). Recreational anglers gather along streams with seines to harvest the fish during their spring spawning runs, cooking them up for the popular ‘smelt fry.’

CONTROLLING SMELT: The spread of rainbow smelt to inland lakes may be due in part to anglers who put live smelt in bait buckets where eggs and milt mixed, fertilized, and then were unintentionally introduced when the angler dumped the contents into the water. Such dumpings are illegal, and since smelt are classified as "rough fish," they cannot be transported live within the state without a permit from the Department of Natural Resources. Current control techniques can't eliminate rainbow smelt from lake communities without harming the rest of the lake population, thus it is most important to educate water users and to always follow the four-step protocol for preventing the spread of aquatic invasives.

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Last Revised: September 3, 2004