Estimating multi-factor cumulative watershed effects on fish populations with an individual-based model

While the concept of cumulative effects is prominent in legislation governing environmental management, the ability to estimate Cumulative effects remains limited. One reason for this limitation is that important natural resources such as fish populations may exhibit complex responses to changes in environmental conditions, particularly to alteration of multiple environmental factors. Individual-based models hold promise for estimating cumulative effects in these situations. We present an example application of an individual-based model of stream trout to the problem of estimating the cumulative effects of multiple environmental changes: elevated wet-season turbidity, elevated dry-season stream temperature, and reduced pool frequency. Each of these physical changes had multiple consequences for individual fish in the model, reflecting existing information. The simulations exhibited non-linear and non-multipticative population responses to the multiple stressors. The results indicate the value of the individual-based approach for estimating cumulative effects and challenge the assumption that consequences for animal populations of increasing or Multiple environmental changes are readily estimated from responses to modest changes in single factors.