Author: Reem Hajjar
Common Name: Ruffe (Eurasian ruffe, river ruffe, pope)
Scientific Name: Gymnocephalus cernuus
Classification:Phylum or Division: Chordata
Ruffe are small, reaching up to 20 cm in length, with olive brown coloring on the back and pale sides. It has spiny dorsal and anal fins. Ruffe resemble young walleye, yellow perch, and trout perch, but can be distinguished from these by its slightly down turned mouth, its large and spiny dorsal fin, and the lack of scales on its head.
The ruffe is native to all of Europe except for along the Mediterranean, western France, Spain, Portugal, parts of Scandinavia, and parts of Great Britain. In Asia, ruffe distribution extends from the Arctic Circle to the Black Sea.
Ruffe are currently spreading throughout the southwestern regions of Lake Superior, and have also been found in Lake Superior's northern coast near Thunder Bay, Canada, and in Lake Huron at the mouth of Thunder Bay River, Michigan (click on map for an enlarged view). Ruffe have also been introduced to several European water bodies outside of its native range in Scotland, Italy, North Wales, England, Norway, Germany, France, Switzerland, and Croatia.
Site and Date of Introduction:
It is unclear when the ruffe first appeared outside of its native range in Europe. The first specimens were collected in North America in 1986, in the St. Louis River Estuary of Minnesota, at the westernmost point of Lake Superior.
Mode(s) of Introduction:
The introduction of the ruffe to North America was unintentional. It is speculated that it was first introduced through ballast water discharged from transoceanic ships. The potential for range expansion through repeated introductions around the Great Lakes region is high, due to the heavy shipping traffic between Lake Superior ports and other Great Lakes ports.
Reason(s) Why it has Become Established:
The ruffe’s eurytopic characteristics have played a large role in its successful invasion of Lake Superior. It has been described as a prolific breeder and aggressive feeder. Its indiscriminate habitat requirements and r-selected life history traits have been conducive to invasion.
Non limiting Habitat Requirements
The ruffe can tolerate a wide range of ecological and environmental conditions. It is found in fresh and brackish water (with salinity up to 12ppt), lacustrine and lotic systems, montane and submontane lakes, and oligotrophic and eutrophic lakes. In the Netherlands, it is found in lakes, big and small rivers, estuaries, and ponds. It occurs at depths varying from 0.25m to 85m. Ruffe increase in abundance with increasing eutrophication, and proliferate with anthropogenic additions of nutrients.
Eurytopic Life History and High Reproduction Rates
Typical of an r-selected species, reproductive potential of the ruffe is exceptionally high. It is highly fecund, matures early, and has a low length at maturity. Ruffe mature in two to three years, but males in some populations may mature in one year in warmer waters, reaching 11-12 cm in length at maturity. Ruffe 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. Females produce up to 200,000 eggs in the first batch, and up to 6,000 eggs per subsequent batch. Eggs hatch in 5-12 days. Young ruffe tolerate temperatures ranging from 7- 30 degrees Celsius. Females generally live for a maximum of 11 years, males for 7 years.
Adaptable Ecological Role
The ecological role played by the ruffe is highly flexible. There is much zonal, local and regional variation in the ruffe diet. Its lack of predators, and ability to thrive in eutrophic conditions have resulted in its becoming established as a fierce competitor.
Depending on its life history stage and location, ruffe 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 ruffe will eat some small fish, including juvenile smelt, gobies, perch and nine-spined sticklebacks.
Ruffe have few predators in Europe and Asia, and most will only prey on ruffe 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.
Ruffe compete with benthivorous fish, including bream, Coregonus spp., roach, sturgeon, smelt, trout perch, Eurasian perch, and yellow perch. However, ruffe may have a competitive advantage due to its flexible foraging abilities. Ruffe thrive in eutrophic conditions such as those associated with human disturbance, out-competing fish with narrower ecological requirements.
The benefits associated with the ruffe invasion of the Great Lakes are limited. A ruffe fishery would have minor commercial value. In some eastern European countries it is considered a delicacy, but is generally only used as a bait by anglers. It has no value as an aquarium fish.
The threats associated with the ruffe introduction greatly outweigh the benefits. Ruffe 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. When introduced to Loch Lomond, Scotland, in 1982, the population grew exponentially, and heavily predated upon salmonid eggs. Between 1989 and 1996, in the St. Louis River Estuary, densities of several native fish decreased as ruffe densities increased. There is a fear that ruffe will likely inhabit larger areas of the lower Great Lakes, which are all shallower and warmer than Lake Superior, leaving ova of several commercially important fish such as the herring, whitefish, and lake trout vulnerable to predation. In addition, it is feared that abundance of the highly valued recreational yellow perch may decrease dramatically, inasmuch as laboratory studies show that ruffe and yellow perch will likely prefer similar food resources where they co-occur.
Control Level Diagnosis:
Control of the spread of the ruffe should be considered a high priority. While its impact 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. Its tolerance of different habitats and environmental conditions ensures successful introduction to novel locations. Its early maturation and high fecundity result in quick increases in abundance and quick establishment. And with the convenient mode of transportation of ballast water in ships traversing the Great Lakes, it is likely that the ruffe will invade further habitats in the Great Lakes. Therefore, it is advisable that the increase and spread of the ruffe is controlled before negative consequences are felt throughout the Great Lakes.
Many physical, chemical, and biological methods have been suggested to contain the spread of the ruffe. In some Polish lakes, stocking of elvers and measures taken to protect pikeperch and eel resulted in an unintentional 5-7 fold decrease of ruffe. However, in 1989, attempts to control ruffe 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 burgeoning ruffe population. Piscicides such as 3-trifluoromethyl-4nitrophenol (TFM) are now being suggested as an effective measure to control the ruffe population, but effects of such chemicals on other biota are questionable.
The Ruffe Task Force was appointed by the Great Lakes Fishery Commission in 1991 to control what has come to be known as “a threat to North American fisheries”. Many believe that it is too late for eradication of the ruffe, 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. The program reached limited success, until ruffe specimens were found in Thunder Bay River, Michigan in 1995.
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