INDIVIDUAL-BASED MODELING OF PCBS EFFECTS ON YOUNG-OF-THE-YEAR LARGEMOUTH BASS IN SOUTHEASTERN USA RESERVOIRS

Effects of polychlorinated biphenyls (PCBs) on young-of-the-year (YOY) largemouth bass (Micropterus salmoides) are simulated using an indivi dual-based population model. The model simulates the daily development , growth, and survival of largemouth bass from eggs to the end of thei r first growing season (October 15) in a single, well-mixed box. The m odel was configured for southeastern USA reservoirs, mostly using data from two Tennessee River impoundments. PCBs exposure levels are expre ssed as tetrachlorodibenzo-p-dioxin (TCDD) concentrations in largemout h bass tissue. PCBs effects of increased mortality and reduced growth (via decreased feeding and increased metabolic rate) are imposed on mo delled individuals dependent on specified exposure concentrations. Mon te Carlo simulation is used to reflect variation in model inputs and e xpress model predictions as probability distributions. The following f ive model predictions of largemouth bass were analyzed: number density (number/ha), biomass density (kg/ha), mean length (cm), and mean cond ition factor (K): all on October 15, and survivorship (fraction of egg s surviving to October 15). PCBs concentrations between 0 and 20 ppm w ere simulated. Predicted bass number density and biomass density incre ased with increasing spawner density and for each spawner density, dec reased with increasing PCBs levels. Both number density and biomass de nsity show a leveling off with increasing spawner density, implying de nsity-dependence (due to bass consumption reducing prey densities) was operating. PCBs effects were apparent but small relative to natural v ariation in model predictions. Number density, biomass density, mean c ondition factor K, and survivorship all decreased, and mean length sli ghtly increased, with increasing PCBs levels. Predicted PCBs effects f or a chronically-exposed population (fewer spawners) were less at low exposure levels and greater at high exposure levels than those for an initially-healthy population. Field-based comparisons of YOY densities have a low power for distinguishing PCBs effects from natural interan nual variability. While the direct effects of PCBs were relatively sma ll, PCBs exposure may increase the risk of populations to natural and other anthropogenic stresses. Some caution should be used when short-t erm predictions are used to assess long-term consequences of contamina nt exposure. Accurate prediction of PCBs effects require realistic rep resentation of YOY largemouth bass growth rates and better methods for estimating exposure in nature. (C) 1997 Elsevier Science B.V.