FATHEAD MINNOW (PIMEPHALES-PROMELAS RAFINESQUE) REPRODUCTION IN OUTDOOR MICROCOSMS - AN ASSESSMENT OF THE ECOLOGICAL EFFECTS OF FISH DENSITY

This study was part of a research program designed to evaluate fathead minnow (Pimephales promelas) reproduction in 17-m(3) outdoor microcos ms as a test system to investigate ecosystem-level impacts of toxicant s. Sex ratios of fathead minnow (FHM) adults at stocking were manipula ted to regulate the biomass and numbers of young-of-the-year (YOY) fol lowing reproduction. This study reports the effect of fish densities o btained from six different sex ratios on microcosm ecology that were c ompared to a fishless control during a 4.5-month period. Fish sampled during the study were representative of the actual population. Taxa co llected from FHM stomachs were very diverse (16 groups) representing v arious communities and trophic levels. Bacillariophyta, filamentous al gae, and macrophytes were the dominant food source; however, Crustacea , rotifers, chironomids, and other macroinvertebrates were also import ant. Effects on community structure were investigated using multivaria te discriminant analysis. There were no fish density effects on zoopla nkton or benthic macroinvertebrate community structures. Microcosms wi th high FHM densities had fewer emerging chironomids, and phytoplankto n levels were elevated. Phytoplankton taxa; which discriminated phytop lankton community structure differences between fish biomass groups, w ere also positively correlated with total phosphorus.