Eurasian Ruffe (Gymnocephalus cernuus)
Eurasian ruffe (Gymnocephalus cernuus) may pose a serious threat to aquatic ecosystems and to sport and commercial fishing. This invader may compete with native fish for food and habitat. First discovered in western Lake Superior in 1986, ruffe populations have rapidly increased in the St. Louis River at Duluth-Superior and spread to other rivers and bays along the south shore of western Lake Superior. They have also spread to Thunder Bay, Ontario on Lake Superior, and the Thunder Bay River, Michigan on Lake Huron.
The potential for ruffe to expand their range in North American waters is causing great concern. Anglers can be the first to discover ruffe because these fish are commonly caught by hook and line. Early detection of isolated populations may help slow or prevent the spread of ruffe. Your help to report new sightings and to prevent their spread is vital.
Identify Eurasian Ruffe
The ruffe is a small fish that resembles a yellow perch with walleye markings. In fact, it is a member of the perch family. An adult ruffe is about five to six inches long. It rarely exceeds 10 inches in length. At first glance, ruffe can resemble young walleye, yellow perch, johnny darter, or troutperch. The ruffe is different from other perch because...
- it has a very large dorsal fin, joined together, front and back The front part of this large dorsal fin has 11-16 spines.
- it has a slightly downturned mouth
- it has no scales on its head
Don't mistake ruffe for troutperch. Troutperch are a soft-rayed fish with a single top fin, and are smaller than ruffe.
- Very slimy when handled
- Usually less than 6 inches (15 cm long)
- Perch-like body shape
What You Can Do
- Learn to identify the ruffe.
- Inspect and remove aquatic plants, animals, and mud from boat, motor, and trailer.
- Drain water from boat, livewell, and bilge before leaving any water access.
- Dispose of unwanted live bait and worms in the trash.
- Never dump live fish from one body of water into another.
- If you catch a ruffe (outside the Duluth area of Lake Superior and the St. Louis River estuary), kill it, freeze it, and call the Minnesota Sea Grant Program in Duluth, (218) 726-8712, or the Minnesota Department of Natural Resources in St. Paul, 1-888-MINNDNR or (651) 259-5100, or a local DNR fishery office.
Do not throw it back alive!
Never transport a live Ruffe. States have differing regulation and penalties regarding possession and/or transportation of live or dead ruffe. Know your state statutes.
Effects of Ruffe on Other Species
Ruffe grow quickly, have a high reproductive capacity, and can live in a wide variety of environments. These qualities give populations of ruffe the potential disrupt the delicate predator/prey balance vital to sustaining a healthy fishery.
Based on population size and diet, ruffe should reduce food sources for many fish species in the St. Louis River Estuary. For example, recently hatched yellow perch must consume large amounts of plankton in a fairly short time in order to grow to the next stage. At stage two, yellow perch must eat larger food items, small crustaceans, and insects abundant near the bottom of the water column. Only after passing these two hurdles do yellow perch get big enough to eat other fish. If the ruffe interrupts either growth stage by reducing the food supply just as the yellow perch need it (an ecological bottleneck), the yellow perch population will crash.
Robust ruffe populations mean less food and space in the ecosystem for other fish with similar diets and feeding habits. Because of this, walleye, perch, and a number of small forage fish species may be compromised. Species that might be vulnerable to competition from ruffe include: emerald shiner, yellow perch and troutperch. It might be easy to blame all fish population changes in the St. Louis Estuary on the ruffe, but some changes could be the result of natural fluctuations, fishing pressure, or fisheries management practices.
In Loch Lomond, Scotland, native perch populations declined dramatically when ruffe were introduced. In some Russian waters, the ruffe has harmed whitefish populations by preying heavily on whitefish eggs. In Europe, the ruffe is known to eat other fish's eggs, but its main diet consists of small water insects and larvae found primarily in the bottom (benthic) layer of the water column. In the St. Louis River Estuary, an important hatchery area for many Lake Superior fish, ruffe stomach samples reveal few fish eggs. But the ruffe is an opportunistic feeder and will eat almost anything. While fish eggs do not seem to be part of the ruffe’s regular diet in the St. Louis River Estuary, that does not guarantee fish eggs won’t be part of the ruffe's diet in other North American habitats.
Fisheries managers in Lake Superior first tried to control ruffe by increasing the number of its predators, especially walleye and northern pike. They did this by limiting sport catches of these species, and stocking walleye and northern pike. Results of the predator-stocking program were disappointing.
Researchers analyzed stomach samples of the predators and found very few ruffe in walleye stomachs. Bullheads appear to be the only species that consistently eat ruffe. Research suggests that predators stocked to control ruffe may not eat them because they prefer soft-rayed shiners and small hard-rayed fish like darters and young perch.
Surveys of northern pike stomachs, however, suggest that ruffe may be growing in importance as a food source. Ruffe made up less than one percent of fish eaten by northern pike in 1989. By 1992, the figure had climbed to 15 percent. Poisoning ruffe in some areas was considered, but was ruled out. As one researcher said, "The cost would have been staggering, and it probably would have failed. All it takes is one pair of ruffe to survive and the problem starts all over again."
To keep ruffe from spreading to the other Great Lakes, the Lake Carriers Association developed best management practice guidelines for handling ballast water in Great Lakes ships.
Ruffe in Europe
In Europe, the ruffe generally matures in two or three years, but it may mature in one year in warmer waters. It spawns between mid-April and July, depending on location, water temperature, and preferred habitat. A female ruffe lives an average of seven years, but may live up to 11 years. Males live up to seven years but have an average lifespan of three to five years.
The ruffe's range includes northeastern France, England, the rivers entering the Baltic and White Seas, most of Siberia, and the Baltic Sea. Before coming to North America, the ruffes most recent expansion was to Loch Lomond, Scotland where it may have been responsible for dramatic declines in the local perch population.
Ruffe can thrive in a wide range of temperatures and habitat. The ruffe has a faster first year growth rate than most of its competitors. It starts reproducing at age two or three, but can reproduce after the first year in warmer waters. An average female can produce 13,000 to 200,000 eggs per season.
In Europe, the ruffe is found in fresh and brackish (salinity less than 3-5 ppm) waters and in all types of lakes from deep, cold, and clear to shallow, warm and full of nutrients. In rivers, the ruffe prefers slower-moving water; in lakes, it prefers turbid areas and soft bottoms, usually without vegetation.
Unlike other perch species, the ruffe is more tolerant of murky, nutrient-rich (eutrophic), conditions. Like walleye, the ruffe spends its days in deeper water and moves to the shallows to feed at night.
To avoid predators, ruffe prefer darkness. Although it has poor eyesight, the ruffe’s head has a well-developed system of bone canals that contain sensory organs called neuromasts. Such organs are common among perch species in early life stages, but they tend to atrophy as the fish reach adulthood. In adult ruffe, however, these sensory organs continue to detect water vibrations given off by both predators and prey.
Jensen, D.A. 1995. Eurasian Ruffe WATCH Card. Minnesota Sea Grant Program (X13)
McLean, Mike. 1997. Ruffe: A New Threat to Our Fisheries. Minnesota Sea Grant Program as a Joint Project of the Great Lakes Sea Grant Network, 1993, 1994, 1997 (X07).