Food Habits and Consumption Estimates of Walleyes Stocked as a Tool to Suppress Fathead Minnow Populations in West-Central Minnesota Wetlands

Wetlands in the prairie pothole region of Minnesota provide important wildlife habitat and food resources for waterfowl. The management of wetland ecosystems to exist in a clear-water, macrophyte-dominated state is desired by state and federal waterfowl biologists. Higher macrophyte biomass results in greater habitat complexity and increased food availability for aquatic invertebrates, while likely increasing food availability for waterfowl. However, persistent, highly abundant fathead minnow Pimephales promelas populations often prevent this clear-water state from occurring. The presence of fathead minnows has detrimental effects on macroinvertebrates, zooplankton, water clarity, and thus macrophytes and epiphyton. Therefore, the suppression of fathead minnow populations by two walleye Stizostedion vitreum stocking treatments was assessed. Six wetlands were stocked in mid-May 2001 with 5.6 kg/ha of walleyes (~3.4 kg/ha age-2 walleyes and ~2.2 kg/ha age-3 walleyes) and walleye food habits were determined monthly from mid-May through mid-September 2002. Prey fishes composed greater than 82% of walleye diets on all sample dates at each wetland throughout 2001 and 2002, except one. Due to the lack of a walleye diet shift by mid-September 2002 to invertebrates, I believe that walleye stocking rates implemented in these wetlands during the 2001 and 2002 sample seasons did not substantially reduce prey fish populations. Walleye consumption estimates of fathead minnows only approached fathead minnow production estimates, which indicates that walleye stocking densities were likely too low. Six other wetlands were stocked in mid-May 2001 and 2002 at a rate of 12,000 fry per hectare. Age-0 walleyes were sampled bi-weekly from mid-June through mid-September 2001. Age-0 and age-1 walleyes were sampled monthly from mid-May through mid-September 2002. A generalized diet shift from zooplankton to fish to macroinvertebrates was observed during the 2001 sample season and diets of each walleye cohort remained primarily macroinvertebrates throughout the 2002 sample season. Stocked walleyes quickly reduced the abundance of prey fish populations throughout this study. Although fathead minnow consumption estimates did not exceed fathead minnow production estimates in all fry wetlands; due to the consumption of primarily age-0 fathead minnows by walleyes, the number of fathead minnows that would have become adults and reproduced was substantially reduced and consequently fathead minnow suppression did occur. Thus, stocking age-0 walleyes was an effective biomanipulation tool that substantially reduced prey fish densities, and likely influenced lower trophic levels in these aquatic communities