We sampled and analyzed diets of stocked northern pike and yellow perch (hereafter pike and perch, respectively) in three southern Minnesota (USA) lakes where fish stocking was used as a tool to potentially extend the number of years that rehabilitated lakes exhibit clear-water conditions following water level drawdown or fish toxicant treatments. We also collected and analyzed diets of native and nonnative (common carp) fishes present in the three treatment lakes and a reference lake. Initially, prey fish populations may be low following rehabilitation treatments, so managers have been experimenting with stocking perch as an initial food source to get pike populations through a growth and survival bottleneck. We hypothesized that pike would be highly piscivorous, but we were less certain about the ecological feeding niche of perch and how it compares to other native and nonnative fish in shallow lakes. In total we collected a data set of 388 fish diet samples for eight fish species across four lakes. We selected multiple predictor variables to help us understand patterns among sites and species, or associated with fish length and various site characteristics (e.g., submerged plant abundance and perch catch-per-unit-effort).
Results/Conclusions
Canonical correspondence analysis was implemented on these data sets and 32% of the variation was explained in our diet response matrix. Ordination plots showed that pike were the most piscivorous species, with perch, fathead minnow and golden shiner comprising the most common prey items. Fish length explained the most variability, and was correlated with increased piscivory. Fathead minnow, golden shiner, and black crappie had signature zooplankton diets, while orangespotted sunfish, bluegill, and green sunfish relied most heavily on macroinvertebrates. Yellow perch were sometimes piscivorous, but overall perch tended to have a generalist diet consisting of both invertebrates and fish, and were not correlated strongly with any of the environmental variables in our model. Overall, diets were highly variable among lakes, and included piscivory (including cannibalism), zooplanktivory, and invertivory. Diets were influenced by a number of factors with fish length, species, and lake explaining the most variation. This indicates diet composition can vary significantly among lakes and can be influenced by ecological state (e.g., water quality), prey abundance, and fish species present. Thus, impacts of fishing stocking on shallow lake ecosystems will likely vary from lake to lake depending on baseline conditions.