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Minnesota Department of Natural Resources, Wildlife Research Unit, 102 23rd Street, Bemidji, MN 56601

Fish predation is a major determinant of aquatic invertebrate density and community structure in lakes, but little research has assessed effects of fish on invertebrates in prairie wetlands. Furthermore, potential implications for breeding waterfowl that feed on aquatic invertebrates are poorly understood. Fathead minnows (Pimephales promelas) are commonly stocked in palustrine wetlands and recent studies indicate that invertebrates are important in their diet. I evaluated effects of fathead minnows on wetland invertebrates in western Minnesota by comparing aquatic invertebrate communities in wetlands with and without dense stocked populations of minnows. Palustrine emergent-aquatic bed wetlands were selected for comparison according to a blocked design based on marsh size and depth. Aquatic invertebrates were sampled with submerged horizontal activity traps (AT) during June-October 1991 and June-August 1992. AT catches during both years were dominated by crustaceans, insects, leeches, and snails. Abundance and biomass of insects, crustaceans, and total invertebrates were used in analysis of covariance (general linear models procedures). Total catch of invertebrates was consistently higher in fishless treatments. This overall pattern resulted from higher catches of crustaceans (Cladocera, Copepoda, Amphipoda, Ostracoda, and Conchostraca) in 1991 and higher catches of crustaceans and insects (Coleoptera, Diptera, Hemiptera, Odonata, and Trichoptera) in 1992. AT catches of insects were similar among all study wetlands in 1991, but insect catches from fishless treatments were higher in 1992. Fathead minnows apparently suppressed abundance and biomass of aquatic insects and crustaceans. Fathead minnows appear to have potential to limit availability of wetland macroinvertebrates, thus reducing the suitability of these habitats as feeding areas for breeding waterfowl.