The ruffe, whose scientific name is Gymnocephalus cernuus, is a fish of the Percidae family that is native to Europe and northern Asia. It has become a problematic invasive species in North America since it was introduced sometime in the 1980s. Efforts have been ongoing ever since to eliminate the fish from the lakes and rivers of Canada and the United States.
The ruffe is a small fish, typically growing to about 4 to 6 inches (10 to 15 cm) in length and weighing about 14 ounces (400 grams). The fish is usually brownish-gray with dark spots covering most of the body, and notable physical characteristics include its long dorsal fins that are joined together.
Eurasian ruffe prefer very cold waters, usually between 77 to 86 degrees Fahrenheit (25 to 30 degrees Celsius). The fish is usually seen in deeper waters during the daylight hours, moving to the shallower waters of lakes and rivers at night when it feeds. The fish primarily feeds on fish eggs.
No one knows exactly how this fish became an invasive species. Most likely it was released in the discharged ballast water of transoceanic ships. It first showed up in Lake Superior, but it has since been sighted in both Lake Huron and Lake Michigan. It has also been seen in surrounding rivers.
The ruffe has become a dangerous invasive species because it competes against other fish of the Great Lakes for food. Studies done by environmental agencies have found that areas with high populations of the fish have decreased populations of native fish like yellow perch and emerald shiners. Its population has been kept somewhat in check, however, by larger fish such as bullheads and smallmouth bass, which feed on the ruffe.
Both state and federal government agencies have been working to control the ruffe population before it grows to dangerous levels. Fishermen in Wisconsin are instructed to kill and freeze any ruffe they catch and report the catch to local government agencies.
Control measures to help combat the spread of the ruffe include the use of pheromones to attract and capture the fish. Alarm pheromones are also being used. These special pheromones can have an influence on the fish's behavior, stopping it from migrating to areas where it has not yet spread.
The ruffe is classified as a prohibited invasive species. It is therefore illegal to import, transport, or even possess a live one. Consequently, they are not seen in aquariums or even in fish markets.