John Wiley & Sons, Ltd.

Assessing population‐level effects of zinc exposure to brown trout (Salmo trutta) in the Arkansas River at Leadville, CO

We assessed population‐level risk to upper Arkansas River brown trout (Salmo trutta L.) due to juvenile exposure to zinc. During spring, individuals in the sensitive young‐of‐the‐year life stage are exposed to elevated zinc concentrations from acid mine drainage. We built and used a simple life‐history population model for the risk assessment, with survival and fecundity parameter values drawn from published data on brown trout populations located in the United States and Europe. From experimental data, we derived a toxicity model to predict mortality in brown trout fry after chronic exposure to zinc. We tested sensitivity of risk estimates to uncertainties in the life‐history parameters. We reached five conclusions: 1) Population projections are highly uncertain. A wide range of estimates for brown trout population growth is consistent with the scientific literature. The low end of this range corresponds to an unsustainable population, a physically unrealistic condition due to combining minimum parameter values from several studies. The upper end of the range corresponds to an annual population growth rate of 281%. 2) Excess mortality from zinc exposure is relatively more predictable. Using our exposure‐response model for excess mortality to brown trout fry due to zinc exposure in the upper Arkansas River at the mouth of California Gulch in the years 2000 to 2005, we derived a mean estimate of 6.1% excess mortality (90% confidence interval = 1.6 to 14.1%). 3) Population projections are sensitive to all the parameters that contribute to the onset of reproduction. The weight of evidence suggests that young‐of‐the‐year survival is most important; it is inconclusive about the ranking of other parameters. 4) Population‐level risk from zinc exposure is sensitive to young‐of‐the‐year survival. If young‐of‐the‐year survival exceeds 20 to 25%, then the marginal effect of excess juvenile mortality on population growth is low. The potential effect increases if young‐of‐the‐year survival is less than 20%. 5) Effect of zinc on population growth is predictable despite high uncertainty in population projections. The estimate was insensitive to model uncertainties. This work could be useful to ecological risk assessors and managers interested in using population‐level endpoints in other risk assessments. Integr Environ Assess Manag © 2012 SETAC

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