Size-Limited Predation, Winterkill, and the Organization of Umbra–Perca Fish Assemblages

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Abstract
We compared population densities and size structures of central mudminnows (Umbra limi) occurring with and without yellow perch (Perca flavescens) in six small lakes in northern Wisconsin over a 6-yr period. Our observations suggested that a size-dependent predator–prey interaction was a major mechanism organizing these assemblages. Densities of mudminnows cooccurring with perch averaged 12% of populations in mudminnow-only lakes. Large mudminnows (≥76 mm total length) predominated in populations with perch whereas smaller fish predominated in single-species assemblages. Following severe winterkills of adult perch in two lakes, mudminnow densities increased significantly due to high survival and recruitment of yearlings. Increases in small mudminnows lasted only a year: densities and size distributions returned to prewinterkill patterns when perch that had survived winterkill attained lengths >13 cm. To examine the role of size-limited predation, we performed an experiment in 2000-L tanks, varying the initial size distributions of mudminnows and perch and measuring size-specific survival among mudminnows. Predation in the experimental populations was greatest when large perch (121–180 mm) were with populations of predominantly small mudminnows (<76 mm); smaller perch (114–132 mm) were significantly less successful as predators whereas large mudminnows were significantly less vulnerable to predation by even large perch. Despite this interaction, central mudminnows coexist at low densities with yellow perch; possible factors permitting coexistence are fish sizes and spatial and structural refuges.