Fish ecology in severe environments of small isolated lakes in northern Wisconsin (abstract)
We and our colleagues have been studying the community, population, behavioral and physiological ecology of fishes in small isolated lakes, many of which have severe environments. We classified the lakes into five types based on their fish assemblages: PIKE, BASS, MUDMINNOW-MINNOW, MUDMINNOW-PERCH, and FISHLESS lakes. Assemblage type can be accurately predicted with a discriminant model containing five environmental variables: lake area, maximum depth, conductivity, pH, and geographic isolation. Low winter oxygen and low pH are important filters of species structure in MUDMINNOW lakes; pH is also important in BASS lakes. Predation appears important in PIKE and BASS lakes where small species without spines are eliminated. Adaptations of species that persist in lakes with low winter oxygen include use of oxygenated microhabitats within the lake, emigration to ·outlet streams, small body size and reduced locomotory activity, and breathing gaseous supplies of oxygen beneath the ice. Species also differ greatly in pH tolerance. Ecotypes of yellow perch, more resistant to low winter oxygen and to low pH than perch from more benign environments, occur in the low oxygen or low pH lakes; differences among populations appear to be both genetic and acclimatory. Perch both compete for food with and prey on central mudminnow in MUDMINNOW lakes; both interactions contribute to tbe numerical dominance of perch over mudminnow. Mudminnow are higher in abundance in lakes where perch are absent or after a partial winterkill of perch. In MUD MINNOW -MINNOW lakes symbiotic relationships between mudminnow and muskrat and between brook stickleback and mudminnow provide sources of winter oxygen. These small insu lar environments provide an excellent theater at which structuring mechanisms of fish assemblages can be observed; their small size and isolation appear to intensify both the effects of severe habitat factors and species interactions relative to larger, less isolated, and more hetergeneous habitats.
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