Assessment of Population Relevance of Endocrine-sensitive Apical Endpoints in Fish Chronic Studies Using Individual-Based Models

Population models have long been thought of as a suitable approach for assessing the population relevance of chemical effects observed on individuals in laboratory studies, although they have rarely been applied in a regulatory context. We modeled potential population-level responses of individual-level adverse effects induced by endocrine-disrupting chemicals (EDCs). We imposed three effect durations (10-year, 3 months summer, or winter) for six common EDC endpoints (fecundity, fertilization rate, sex ratio: male and female skew, courtship and nesting behavior) at four magnitudes of effect (10, 20, 50 and 90% reduction) using individual-based population models for three fish species with differing life histories: stickleback (Gasterosteus aculeatus), brown trout (Salmo trutta) and zebrafish (Danio rerio). The suitability of different assessment criteria for determining the significance of population responses was evaluated. For all endpoints tested individually, effect magnitudes of 20% did not result in any population-level responses in each of the three species, except in the stickleback, where a 20% reduction in fecundity or fertilization rate led to population declines (in these cases, effect magnitudes of 10% did not result in population-level responses). Once standardized, our "exposure-agnostic molecule-independent" approach will enhance our understanding of population outcomes within the regulatory hazard-based assessment of EDCs.