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 (<i>Gasterosteus aculeatus</i>), brown trout (<i>Salmo trutta</i>) and zebrafish (<i>Danio rerio</i>). 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.