Relative abundance and distribution of ruffe (Gymnocephalus cernuus) in a Lake Superior coastal wetland fish assemblage

Fish assemblages from Allouz Bay Wetland in the St. Louis River estuary were sampled with fyke-nets from May to October, 1995, to characterize typical use patterns in different seasons and microhabitats. The relative abundance and distribution of ruffe (Gymnocephalus cernuus) in these habitats was of interest because their recent invasion into the Great Lakes has the potential to disrupt native fish assemblages. A total of 15,867 fish comprised of 34 species were captured in 2,300 h of netting. The majority of fish over the whole study were caught in the outer marsh (63%, 9,957 individuals), and seasonally during late June (7,384 individuals/4 net-nights) and early May (2,281 individuals). Yellow perch (Perca flavescens), brown bullhead (Ameiurus nebulosus), emerald shiner (Notropis atherinoides), and silver redhorse (Moxostoma anisurum) were the most abundant species, comprising 85 percent of the total catch. Ruffe was the seventh most abundant species captured (294 individuals), comprising only two percent of the total catch. They were the fifth most abundant species in the outer marsh, but only thirteenth most abundant in the inner marsh. Ninety-one percent of all ruffe (268 individuals) were caught in the outer marsh. Of the 75 species by life-stage combinations derived by classifying all individuals captured into one of 3 life stage categories (YOY, yearling, and adult), yearling ruffe were the twelfth most abundant, adult ruffe were sixteenth, and YOY ruffe were twenty-seventh. While ruffe have been the most abundant fish captured in bottom trawls in St. Louis River estuary during the 1990s, our results indicate the invasion of ruffe in shallow, heavily vegetated areas like those in Allouez Bay has been much less successful. Our results also suggest further degradation of coastal wetlands and other vegetated habitats would eliminate significant refugia from ruffe competition and could lead to increased dominance of ruffe in shallow water habitats in the Great Lakes.