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Modeling effects of toxin exposure in fish on long-term population size, with an application to selenium toxicity in bluegill (Lepomis macrochirus)
A primary goal in ecotoxicology is the prediction of population-level effects of contaminant exposure based on individual-level response. Assessment of toxicity at the population level has predominately focused on the population growth rate (PGR), but the PGR may not be a relevant toxicological endpoint for populations at equilibrium. Equilibrium population size may be a more meaningful endpoint than the PGR because a population with smaller equilibrium (i.e., long-term mean) size is more susceptible to the negative effects of environmental variability. I address the ecotoxicology individual-to-population extrapolation problem with modeling. I developed and analyzed a general model applicable to many freshwater fish species that includes density-dependent juvenile survival and additional juvenile mortality due to toxicity exposure, and I quantified its effect on equilibrium population size as a means of assessing toxicity. I then used selenium toxicity in bluegill sunfish as an example to assess the effects of environmental stochasticity on toxicity response with simulation modeling. Individual-level effects are typically greater than population-level effects until the individual effect is large, due to compensatory density-dependent relationships. These effects are sensitive to the recruitment potential of a population, in particular the low-density first-year survival rate S_b. Assuming high S_b could result in underestimating effects of population-level toxicity. The equilibrium size depends directly on S_b, the reproductive potential, the toxin concentration at which mean mortality is 50% (LC50), and the rate at which individual mortality increases with increasing toxin concentration. More experimental data are needed to decrease the uncertainty in estimating these parameters. Effects of environmental variability resulted in simulated extinctions at much lower toxin concentrations than predicted deterministically.