Gymnocephalus cernuus are small, reaching up to 20cm in length, with olive brown colouring on the back and pale sides. They have spiny dorsal and anal fins (Hajjar, 2002).
According to Hajjar (2002), \"Gymnocephalus cernuus have few predators in Europe and Asia, and most will only prey on G. cernuus when other prey is scarce. Predators include pike perch, northern pike, some eel, burbot, lake trout, small-mouth bass, black crappie, bullheads, walleye, Eurasian perch, yellow perch, cormorants, and kingfishers. To avoid predators, the ruffe prefers darkness, and uses special sensory organs called \"neuromasts\" to detect predators and prey. The ruffe also has a large, spiny dorsal fin likely unpalatable to predators.
According to Hajjar (2002), “the reproductive potential of G. cernuus is exceptionally high. It matures early, in two to three years, but males in some populations may mature in one year in warmer waters, reaching 11-12cm in length at maturity. G. cernuus spawn in a variety of habitats and environmental conditions. Spawning occurs at a wide range of temperatures, 4.9 to 20 degrees Celsius, and on a variety of substrates, including submerged plants, logs, branches, gravel, rocks, hard bottoms of clay, and sand. Eggs develop normally at pH 6.5 to pH 10.5, one of the widest ranges from a broad set of fish tested. Eggs hatch in 5-12 days. Young tolerate temperatures ranging from 7- 30 degrees Celsius. Females generally live for a maximum of 11 years, males for 7 years.”
Hajjar (2002) explains that Gymnocephalus cernuus fishery would only have minor commercial value. \"In some eastern European countries it is considered a delicacy, but is generally only used as a bait by anglers.\"
G. cernuus can tolerate a wide range of ecological and environmental conditions. They are found in fresh and brackish water (with salinity up to 12ppt) and occur at depths varying from 0.25m to 85m. they are also able to thrive in eutrophic conditions (Hajjar, 2002).
According to Hajjar (2002), G. cernuus is a prolific breeder. \"Females produce up to 200,000 eggs in the first batch, and up to 6,000 eggs per subsequent batch.\"
Hajjar (2002) writes that “Gymnocephalus cernuus is an aggressive feeder. Depending on its life history stage and location, G. cernuus prey upon rotifer and copepod nauplii, cyclopoid copepods, cladocera, and chironomid larvae, macrocrustaceans, heleids, dragonfly and caddisfly larvae, zooplankton, mollusks, water mites, isopods, fly larvae and juveniles, and fish larvae (especially Coregonus spp.), and smelt. Larger G. cernuus will eat some small fish, including juvenile smelt, gobies, perch and nine-spined sticklebacks. According to Sea Grant (2002) \"G. cernuus spends its days in deeper water and moves to the shallows to feed at night.\" Nutrition includes eggs of whitefish (Coregonus spp.) (Adams & Maitland, 1998); C. lavaretus ( Schmid, 1998) and C. albula (Winfield et al. 2004).
Introductions occur through escaped or discarded live bait.According to Hajjar (2002), the fish was introduced and is mainly spread by boats taking on ballast water and bring it to new locations.
Principal source: Hajjar, R. 2002. Ruffe (Gymnocephalus cernuus) Columbia University , New York, United States.
Compiler: National Biological Information Infrastructure (NBII) & IUCN/SSC Invasive Species Specialist Group (ISSG)
Review: Ian J Winfield. Centre for Ecology & Hydrology, UK
Publication date: 2006-03-31
Recommended citation: Global Invasive Species Database (2021) Species profile: Gymnocephalus cernuus. Downloaded from http://www.iucngisd.org/gisd/species.php?sc=544 on 17-01-2021.
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.\"
According to Hajjar (2002), many physical, chemical, and biological methods have been suggested to contain the spread of the G. cernuus. In some Polish lakes, stocking of elvers and measures taken to protect pikeperch and eel resulted in an unintentional 5-7 fold decrease of G. cernuus. In 1989, attempts to control G. cernuus with a top-down predator control strategy failed. Northern pike and walleye were stocked in problem areas, but both species preferred native species and failed to control the G. cernuus population. Piscicides such as 3-trifluoromethyl-4nitrophenol (TFM) are now being suggested as an effective measure to control the G. cernuus population, but effects of such chemicals on other biota are questionable. Many believe that it is too late for eradication of G. cernuus, and instead are concentrating efforts on controlling the spread of the invasion. The Volunteer Ballast Water Management Program was enforced in 1993 for this purpose. In this joint effort among several Canadian and American coastal organizations and ocean lines, ships calling at ports in the western portions of Lake Superior were discouraged from taking on or discharging ballast water from these areas.\"\" According to Sea Grant (2002), \"fisheries managers have also considered a program to net and destroy as many ruffe as possible in the St. Louis River, on the theory that the ruffe's range would not expand as rapidly if populations were controlled.\" A modified design of the Windermere trap, (a useful collapsible tool in surveillance, monitoring, or control programs for ruffe or similar species, especially in situations where gillnetting or bottom trawling are not feasible) is found to be inexpensive and lightweight.