Ruffe: A New Threat to Our Fisheries

The ruffe (pronounced rough), is a small but aggressive fish species native to Eurasia. It was introduced into Lake Superior in the mid-1980s in the ballast water of an ocean-going vessel.

Because the ruffe grows very fast, has a high reproductive capacity, and adapts to a wide variety of environments, it is considered a serious threat to commercial and sport fishing. It also has the potential to seriously disrupt the delicate predator/prey balance vital to sustaining a healthy fishery.

Effects of the Ruffe on other species

Explosive growth of the ruffe population means 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 are seriously threatened by continued expansion of the ruffe’s range.

While it is too early to tell exactly how the ruffe will affect other fish in the St. Louis River and Duluth/Superior harbor area of Lake Superior, its numbers have increased dramatically while other species, especially emerald shiner, yellow perch and troutperch, have declined. It would be easy to blame all of these changes on the ruffe, but some could be the result of natural fluctuations, fishing pressure, or fisheries management practices.

How fast is the Ruffe spreading

The ruffe was first collected in the Duluth/Superior harbor area of Lake Superior in 1986 during a routine analysis of the local fishery. The first official identification of ruffe was in 1987, but it was probably introduced around 1985. In the short time since its introduction, the ruffe has become the most numerous fish in the St. Louis River, as measured by trawl samples. As of 1993, the ruffe had spread east along Lake Superiors coast to Chequamegon Bay in northern Wisconsin, and north to Thunder Bay, Ontario. Ruffe probably moved across the lake to Thunder Bay via intralake ballast exchange. So far, Lake Superior is the only place the ruffe is found in the Western Hemisphere. Ruffes ability to move from lake to lake in ships ballast, however, will make it difficult to prevent the fish from expanding its range to the lower Great Lakes.

Control strategies in the St. Louis River

Fisheries managers first tried to control ruffe by increasing the number of it’s predators, especially walleye and northern pike. They did this by limiting sport catches of these species, and stocking walleye and northern pike. Early results of the predator stocking program have been disappointing, but it is too early to judge the effectiveness of this approach since fish often take several years to switch to a new food source.

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. This could explain the increase in ruffe and reduction in these forage species.

Recent 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.

The battle to keep the ruffe from spreading is being fought on several fronts. For instance, poison may be used to eradicate ruffe when the fish are found in small numbers at a new location. Poisoning was considered for areas where the ruffe is firmly entrenched, 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.”

Chemical controls that kill ruffe but leave other species unharmed are being sought. For instance, researchers are exploring the possibility that the ruffe is susceptible to low doses of the lampricide TFM, a chemical that in low doses kills lamprey but not other fish. Recent field tests have shown that treating streams with TFM for lamprey control kills a high percentage of the ruffe. TFM, however, is only registered for use on lamprey. 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 ruffes range would not expand as rapidly if populations were controlled.

Fisheries managers will plan eradication and control measures for Lake Superior rivers and streams on a case by case basis. The overall goal, however, is to contain ruffe to the western part of Lake Superior. To keep ruffe from spreading to the other Great Lakes, the Lake Carriers Association developed voluntary guidelines for handling ballast water in Great Lakes ships. Under these new guidelines, ships going to other Great Lakes are required to exchange ballast in deep (at least 240 feet) water west of a demarcation line between Ontonagon, Michigan and Grand Portage, Minnesota, as well as at least five miles from the south shore of Lake Superior.

What do we know about the Ruffe?

Because ruffe are so new to North America, fisheries managers rely on European studies that describe their life cycle and habits. Even with this data however, it is difficult to predict how an exotic species will act in a new environment.

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 ruffes 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.

A Hearty species

The 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. Due in part to its hearty reproductive rate, ruffe populations can explode quickly.

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, the ruffe prefers 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.

Do Ruffe eat other fish’s 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, 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. So far, the ruffe seems to have the same basic diet of insects and larvae it has in it’s native Eurasian environment. While fish eggs do not seem to be part of the ruffe’s regular diet in the St. Louis River, that does not guarantee fish eggs won’t be part of the ruffe’s diet in other North American habitats.

The “Bottleneck Effect”

Whether ruffe feed on fish eggs may be less important than whether ruffe eat the food preferred by other fish. Because of its sheer numbers and the variety of food it eats, the ruffe will reduce food sources for many fish species. If the ruffe, in turn, is not eaten by native predators, or if young predator fish starve before they get big enough to eat ruffe, the result will be an explosion of ruffe and population bottlenecks in other species.

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.

What you can do to slow the spread of the Ruffe

Because ruffe could easily spread to inland waters, the Minnesota and Wisconsin departments of natural resources enforce regulations that make it illegal to transport ruffe, dead or alive. The ruffe cannot be used as bait by anglers, and bait dealers who trap in areas infested with ruffe should take special precautions to insure they are not contributing to the spread of this exotic species.

Profile view of Ruffe

The ruffe is one of a number of exotic species causing concern in the Great Lakes region. You can help prevent the spread of exotic species with a thorough inspection of your boat and fishing gear. Clean all mud and plant matter from your boat, trailer, propeller, live well and anchors before leaving the landing. Even canoes should be checked inside and out.

Drain live wells, bait buckets, and bilge areas. Inspect and clean fishing tackle. Let your boat and trailer dry in the sun for at least three days before your use it again in another lake or river. Otherwise, wash your boat with very hot water (at least 105 degrees F).

Ruffe are a threat to fisheries

Predicting the effect of any exotic species in it’s new home is difficult. The ruffe is no exception. Experience in Scotland and Russia, however, points to serious problems for North American fisheries if ruffe escapes from its limited range in western Lake Superior. In Loch Lomond, Scotland, native perch populations declined dramatically when the ruffe was introduced. In some Russian waters, the ruffe has harmed whitefish populations by preying heavily on whitefish eggs. Movement of the ruffe to the lower Great Lakes, or to inland lakes and rivers, could have devastating consequences.

What does a Ruffe look like?

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, but there are ways to tell the difference.

The most obvious differences are the ruffes large, continuous dorsal fin and its slightly downturned mouth. Other distinguishing characteristics:

  • size: ruffe are small, adults average 4-6 inches in length
  • color: color and markings are similar to walleye. The average ruffe is 4-6 inches long The ruffe is a member of the perch family, so…
  • it has spiny dorsal and anal fins
  • it has two fins on top, the front fin has hard and sharp spines (also called rays), the back fin has soft rays.

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 softrayed fish with a single top fin, and are smaller than ruffe.

Credits:

Written by Mike McLean, Minnesota Sea Grant communications coordinator, with technical assistance from Derek Ogle, University of Minnesota Department of Fisheries, and Jeffrey Gunderson, Minnesota Sea Grant extension educator.

Troutperch illustration: copyright 1975, Dr. W. B. Scott, used with permission.

Yellowperch illustration: copyright 1957, Samuel Eddy, Wm. C. Brown Publishers, used with permission.

Ruffe illustration: Copeia 1974. No. 2

Graphs created from information found in Trophic Relations of Ruffe (Gymnocephalus cernuus) in the St. Louis River Harbor, Lake Superior by Derek Ogle, June 1992.

Produced by the Minnesota Sea Grant Program as a Joint Project of the Great Lakes Sea Grant Network, Spring 1993, update: Fall 1994

Minnesota Sea Grant is a statewide program that supports research, extension, and educational programs related to Lake Superior and Minnesotas water resources. Sea Grant offices are located on the Duluth campus of the University of Minnesota. Sea Grant is funded by the National Oceanic and Atmospheric Administration through the US Department of Commerce; the state legislature; and the University of Minnesota.

The Great Lakes Sea Grant Network is a cooperative program of the Illinois-Indiana, Michigan, Minnesota, New York, Ohio, and Wisconsin Sea Grant programs. Sea Grant is a university-based program designed to support greater knowledge and wise use of the Great Lakes and ocean resources. Through its network of advisory agents, researchers, educators, and communicators, the Great Lakes Sea Grant Network supplies the region with usable solutions to pressing problems and provides the basic information needed to better manage the Great Lakes for both present and future generations. Sea Grant is in the National Oceanic and Atmospheric Administration (NOAA), Department of Commerce.

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