Fathead minnow (Pimephales promelas rafinesque) reproduction in outdoor microcosms: An assessment of the ecological effects of fish density

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Abstract
This study was part of a research program designed to evaluate fathead minnow (Pimephales promelas) reproduction in 17‐m3 outdoor microcosms as a test system to investigate ecosystem‐level impacts of toxicants. Sex ratios of fathead minnow (FHM) adults at stocking were manipulated to regulate the biomass and numbers of young‐of‐the‐year (YOY) following reproduction. This study reports the effect of fish densities obtained from six different sex ratios on microcosm ecology that were compared to a fishless control during a 4.5‐month period. Fish sampled during the study were representative of the actual population. Taxa collected from FHM stomachs were very diverse (16 groups) representing various communities and trophic levels. Bacillariophyta, filamentous algae, and macrophytes were the dominant food source; however, Crustacea, rotifers, chironomids, and other macroinvertebrates were also important. Effects on community structure were investigated using multivariate discriminant analysis. There were no fish density effects on zooplankton or benthic macroinvertebrate community structures. Microcosms with high FHM densities had fewer emerging chironomids, and phytoplankton levels were elevated. Phytoplankton taxa, which discriminated phytoplankton community structure differences between fish biomass groups, were also positively correlated with total phosphorus.