General Impact
According to Sea Grant (2002), "Gymnocephalus cernuus compete with native fish for food and habitat. Because of this, walleye, perch, and a number of small forage fish species are seriously threatened by continued expansion of the ruffe's range. Hajjar (2002), describes G. cernuus as prolific breeders and aggressive feeders. Their indiscriminate habitat requirements and selected life history traits are conducive to invasion. Their tolerance of different habitats and environmental conditions ensures successful introduction to novel locations.Their early maturation and high fecundity result in quick increases in abundance and quick establishment. G. cernuus have a competitive advantage over other bottom feeding fish, such as bream, Coregonus spp., roach, sturgeon, smelt, trout perch, Eurasian perch, and yellow perch, due to their flexible foraging abilities. They also “thrive in eutrophic conditions such as those associated with human disturbance, out-competing fish with narrower ecological requirements. They have been implicated in density declines of native fish by egg predation and competition for food in some European waters where they have been introduced. While the impact of G. cernuus on the Great Lakes ecosystem has not yet been considerable, the population is increasing and spreading, and has the potential to detrimentally effect highly valued commercial fishery species throughout the Great Lakes. And with the convenient mode of transportation of ballast water in ships traversing the Great Lakes, it is likely that G. cernuus will invade further habitats in the Great Lakes."
Location Specific Impacts:Upper Lake Constance (Europe) 
Predation: Ova predation by ruffe may have negative effects on the natural recruitment of nearshore spawning Coregonus lavaretus (Schmid, W., 1998) Lake Superior (North America)
Competition: Data from this study indicates the ruffe may be negatively impacting brown bullhead and yellow perch by competing for food and habitat and causing a competitive decline in spottail shiner, a preferred food for those species.
Predation: A study (Segelby, 1998) which examined the food habits, (especially the predation of fish eggs) of ruffe in the St. Louis River estuary, Lake Superior showed that the ruffe predation could play a role in overwinter mortality for fall spawning fish in the Great Lakes. The stomach contents analysis showed the presence of eggs of the lake herring (Coregonus artedii). Loch Lomond (United Kingdom (UK))
Predation: There are concerns about increased ruffe predation on existing species especially the powan Coregonus lavaretus (Maitland and East, 1989). Great Lakes (USA) (United States (USA))
Competition: The authors of this study Fullerton et al 2001, conclude that the invasion of G. cernuus, an exotic percid from Eurasia that is now established in limited areas of the Great Lakes, is detrimental to the growth of the native yellow perch Perca flavescens, whose niche it occupies.
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