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“Bone-Cold Caf” Suits Ruffe

The ambiance is intense: dark...cold...resonating with eerie sounds.

The main entre is benthos. It’s not fine dining, but today an estimated six million Eurasian ruffe are grappling for the biggest and the best items on the menu at the bottom of the Duluth-Superior Harbor.

The non-native ruffe were found among the bottom-dwelling fish of the Duluth-Superior Harbor in 1986. Since then, they have become the most abundant fish trawled from the harbor and a competitive threat to native yellow perch and other benthic fish. Sea Grant researchers Ray Newman, associate professor with the University of Minnesota Department of Fisheries and Wildlife, and graduate student Fred Henson, now with the Minnesota Department of Natural Resources in St. Paul, highlight one of the reasons ruffe are thriving in the 17 degrees Celsius (63 degrees Fahrenheit) water in a journal article published in the March 2000 issue of Transactions of the American Fisheries Society.


The jury’s still out about whether Eurasian ruffe have a detrimental impact on yellow perch, but this research project shows there’s another cause for concern.

Henson and Newman conducted a series of laboratory experiments that suggest when adult ruffe taken from the Duluth-Superior Harbor are fed amounts of food similar to what they consume in the field, they convert food to energy most efficiently when the temperature is about 14C (57F). “For reference,” said Minnesota Sea Grant’s fisheries expert Jeff Gunderson, “optimal growth for adult yellow perch occurs at about 23C (73F) and perch don’t grow in water that is colder than 12.5C (54.5F).”

Temperature affects the growth of ruffe less than perch, according to the researchers. “One of the key issues, in comparison to published research on perch, is that ruffe are less efficient at processing food,” said Newman. “More food is needed to sustain a given biomass of ruffe than the same biomass of perch. Ruffe leave a bigger ecological footprint and are more likely to significantly alter the benthic forage base.”

With a few exceptions, fish are approximately the same temperature as the water that surrounds them. Like the speed of molasses, enzymatic reactions allowing fish to process food and maintain their bodily systems and functions tend to slow down at colder temperatures. The combination of enzymes found in a species of fish generally operate most effectively at temperatures similar to those in the environment where they evolved. Ruffe are native to Europe and Siberia and, as indicated by Henson and Newman’s research, tend to fare well in water from 8C to 23C (46F to 73F).

When food is a limiting resource, as it appears to be in the harbor, Henson and Newman suggest that ruffe convert food to energy more efficiently at lower temperatures. According to their research, a ruffe would gain weight on a diet equaling four percent of its body weight at 14C but lose weight on the same diet if the water temperature rose to 23C.

Despite the ruffe’s additional food requirements for adult growth, it is a temperature generalist that can forage efficiently in cooler water and attain sexual maturity sooner than perch. These characteristics give it an ecological advantage in the Duluth-Superior Harbor, a theory supported by the 1999 trawling efforts of U.S. Geological Survey researchers, who caught 27 ruffe for every perch. Henson and Newman conclude their paper by suggesting that the ruffe’s ability to devour benthic prey and to grow in the harbor during the colder months enhances its fitness and magnifies its ability to compete with other species of fish.

To order, look on the journal reprint publications page and our order form, under item JR 454.

By Sharon Moen
June 2000

Return to June 2000 Seiche

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