SELECTIVE PREDATION BY MUSKRATS ON FRESH-WATER MUSSELS IN 2 MINNESOTARIVERS

Muskrat (Ondatra zibethicus) predation has been hypothesized to influe nce the structure of freshwater mussel assemblages. We compared the sp ecies composition and sizes of mussels collected from middens and in s itu at 3 locations in the St. Croix River and 1 location in the Missis sippi River. Species diversity was greater for mussels collected from the rivers compared to those collected from middens. Logistic regressi on revealed mussel species composition and mussel size differed signif icantly between the rivers and middens, which suggested that muskrats were both size-selective and species-specific feeders. A significant i nteraction between species and size may result from muskrats taking la rge individuals from small species and small individuals from large sp ecies. There was selection for the small species, Truncilla truncata, at the 2 sites where it was the dominant species, and avoidance at the other 2 sites. This result suggested that muskrats may be using a str ategy to maximize energy by choosing prey of a particular size, but in locations where larger prey items are not available they will accept smaller species that may normally be considered a marginal prey.