Population modelling to assess the effects of a copper pesticide on rainbow trout (Oncorhynchus mykiss)

Currently, ecological risk assessment at lower tiers is mostly based on laboratory toxicity tests that measure the effect of pesticides on individual-level endpoints. Mortality, dry weight, biomass, fecundity and hatchability are individual-level endpoints that are often measured in fish toxicity tests. However, individual-level endpoints do not necessarily translate directly to a similar effect at the population level and guidelines based on individual-level endpoints might thus be inadequate for the protection of species at population level. Population models can help to overcome this problem. Therefore, the goal of this study was to extrapolate the effects of a Copper hydroxide WP pesticide on rainbow trout (Oncorhynchus mykiss) from a toxicity test to more realistic exposure scenarios, using a population model. To estimate population level effects, an existing individual-based model for brown trout, inSTREAM-Gen was adapted and extended with a toxicity component. Pesticide effects on survival were incorporated in the model using toxicokinetic-toxicodynamic models e.g. the General Unified Threshold for Survival (GUTS) model. These toxicity models are specifically designed to cope with time-varying exposure concentration, which are typical for pesticides. The population model was used to predict population dynamics over a multi-year simulation period and to assess how a realistic exposure to the pesticide is expected to affect this population. To this end, different exposure scenarios were selected where e.g. the number of pesticide applications and the application period were altered. Effect concentrations were derived for a selection of population-level endpoints (e.g. relative population size, probability of persistence) and critically compared with the effect concentrations derived for the individual level. The use of population models and realistic exposure scenarios offers a chance to extrapolate the effects of laboratory toxicity tests to the field based on solid ecological principles. This approach can serve as an intermediate tier tool that utilizes the results of lower tier tests and avoids the costs associated with higher tier multi-species tests.