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 individual-based population model. The model simulates the daily development, growth, and survival of largemouth bass from eggs to the end of their first growing season (October 15) in a single, well-mixed box. The model was configured for southeastern USA reservoirs, mostly using data from two Tennessee River impoundments. PCBs exposure levels are expressed as tetrachlorodibenzo-p-dioxin (TCDD) concentrations in largemouth bass tissue. PCBs effects of increased mortality and reduced growth (via decreased feeding and increased metabolic rate) are imposed on modelled individuals dependent on specified exposure concentrations. Monte Carlo simulation is used to reflect variation in model inputs and express model predictions as probability distributions. The following five model predictions of largemouth bass were analyzed: number density (number/ha), biomass density (kg/ha), mean length (cm), and mean condition factor (K): all on October 15, and survivorship (fraction of eggs surviving to October 15). PCBs concentrations between 0 and 20 ppm were simulated. Predicted bass number density and biomass density increased with increasing spawner density and for each spawner density, decreased with increasing PCBs levels. Both number density and biomass density show a leveling off with increasing spawner density, implying density-dependence (due to bass consumption reducing prey densities) was operating. PCBs effects were apparent but small relative to natural variation in model predictions. Number density, biomass density, mean condition factor K, and survivorship all decreased, and mean length slightly increased, with increasing PCBs levels. Predicted PCBs effects for 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 interannual variability. While the direct effects of PCBs were relatively small, PCBs exposure may increase the risk of populations to natural and other anthropogenic stresses. Some caution should be used when short-term predictions are used to assess long-term consequences of contaminant exposure. Accurate prediction of PCBs effects require realistic representation of YOY largemouth bass growth rates and better methods for estimating exposure in nature. (C) 1997 Elsevier Science B.V.