FATHEAD MINNOW REPRODUCTION IN OUTDOOR MICROCOSMS - A COMPARISON TO BLUEGILL SUNFISH REPRODUCTION IN LARGE MESOCOSMS

Large-scale aquatic mesocosm studies were formerly required by the U.S . Environmental Protection Agency (EPA) for ecological risk assessment in pesticide registration. However, the use of bluegill sunfish (Lepo mis macrochirus; Rafinesque) to evaluate pesticide effects on fish rep roduction in large mesocosms (>300 m(3)) resulted in over-exploitation of invertebrates by large numbers of young-of-the-year (YOY) bluegill , thus confounding the interpretation of pesticide effects. As an alte rnative, a reproduction study with fathead minnows (Pimephales promela s Rafinesque) was conducted in 17-m(3) outdoor microcosms. The objecti ve of this research was to evaluate the effects of varying sex ratios of 20 adult fathead minnows (FHMs) on their reproduction during a 4.5- month period. The outcome of this study was compared to a bluegill rep roduction study in 400-m(3) mesocosms conducted synchronously. The log istics of monitoring the microcosms were simpler, and harvesting FHMs from microcosms took considerably less effort in comparison to harvest ing bluegills from mesocosms. The FHMs produced a smaller number of la rger young in comparison to bluegills, which produced large numbers of less mature YOY. The variability in total fish biomass between replic ates was greater in the bluegill study, resulting in less statistical power to detect biomass effects. The FHM sex ratios significantly affe cted reproduction and growth, thus altering the fish population struct ure, despite similar stocking densities. Sex ratios with similar numbe rs of males and females produced a lower total biomass and number of Y OY. There was no evidence of system carrying-capacity constraints on F HM growth or reproduction.