A member of the Percidae family of perch, the ruffe was introduced into North America, evidently through ballast water discharge by transoceanic ships. It has become a considerable threat to the delicate predator-prey balance necessary to maintain ﬂourishing ﬁsheries in North American waters, especially in the Great Lakes. It has been reported only in Lake Superior waters but is likely to exist, or spread, elsewhere.
The species found and multiplying in Lake Superior has been identiﬁed as Gymnocephalus cernuus. The native range of G. cernuus is from France to the Kolyma River in eastern Siberia, and it has been introduced to England, Scotland, and Scandinavia.