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   Gymnocephalus cernuus (fish)   
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      Gymnocephalus cernuus (Photo: Gary Cholwek, USGS, www.forestryimages.org) - Click for full size   Gymnocephalus cernuus (Photo: Doug Jensen, Minnesota Sea Grant, www.forestryimages.org) - Click for full size
    Taxonomic name: Gymnocephalus cernuus (Linnaeus, 1758)
    Synonyms: Acerina cemua (Linnaeus, 1758), Acerina czekanowskii, Acerina fischeri, Acerina vulgaris, Perca cermua Linnaeus, 1758
    Common names: blacktail (English), Eurasian ruffe, pope, redfin darter (English), river ruffe, ruffe (English)
    Organism type: fish
    Gymnocephalus cernuus is introduced into new locations in the ballast water of ships. Introductions also occur through escaped or discarded live bait. It has become a threat to the Great Lakes in North America and some lakes in Europe. Gymnocephalus cernuus has become invasive due to its reproduction ablity; its wide habitat range and its aggressive feeding habits.
    Description
    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).
    Similar Species
    Perca flavescens, Percopsis omiscomaycus, Stizostedion vitreum

    More
    Occurs in:
    estuarine habitats, lakes, water courses
    Habitat description
    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).
    General impacts
    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."
    Uses
    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."
    Notes
    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.
    Geographical range
    Native range: Asia and Europe (Hajjar, 2002).
    Known introduced range: Europe and North America (Hajjar, 2002)
    Introduction pathways to new locations
    Contaminated bait: Introductions occur through escaped or discarded live bait.
    Ignorant possession: According to Hajjar (2002), the fish was introduced and is mainly spread by boats taking on ballast water and bring it to new locations.
    Ship ballast water: According to Hajjar (2002), the fish was introduced and is mainly spread by boats taking on ballast water and bring it to new locations.


    Local dispersal methods
    Boat: According to Hajjar (2002), the fish was introduced and is mainly spread by boats taking on ballast water and bring it to new locations.
    Management information
    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.
    Nutrition
    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).
    Reproduction
    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."
    Lifecycle stages
    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.”
    Reviewed by: Ian J Winfield. Centre for Ecology & Hydrology, UK
    Compiled by: National Biological Information Infrastructure (NBII) & IUCN/SSC Invasive Species Specialist Group (ISSG)
    Last Modified: Friday, 31 March 2006


ISSG Landcare Research NBII IUCN University of Auckland