Rainbow Trout (Oncorhynchus mykiss) as a Model Organism for Assessing Long-Term Impacts of Municipal Wastewater Pollution on Early Fish Development.

The increasing release of micropollutants from wastewater treatment plant (WWTP) effluents poses growing risks to aquatic ecosystems; however, linking environmental exposure to biological responses under environmentally realistic conditions remains challenging. This study aimed to evaluate biological responses associated with WWTP effluent exposure during the early life stages of rainbow trout (Oncorhynchus mykiss) using an integrated field-based approach. Five floating incubations containing 1000 eggs each were deployed at each of an upstream reference site and a downstream WWTP-affected site. Mortality, metabolic rate, growth, and development traits, and the concentrations of selected pharmaceuticals and personal care products (PPCPs) in fish and water were analysed regularly throughout development, followed by multi-tissue transcriptomic analyses after five months of release. Fish at the downstream site exhibited higher mortality (56% vs 23%), lower temperature-corrected growth and developmental rates, and greater depletion of energy reserves than fish at the upstream site. Meanwhile, more PPCPs were detected in fish from the downstream site, with generally higher concentrations than at the upstream site. Transcriptomic analyses further revealed coordinated changes in genes and pathways related to energy metabolism, immune regulation, neurotransmission, and oxygen delivery, suggesting molecular responses consistent with the observed physiological alterations. By integrating contaminant occurrence, bioaccumulation, organism-level endpoints, and transcriptomic responses within a single field-based study, this work provides a comprehensive assessment of biological responses associated with environmental conditions at a WWTP-affected site. Our findings suggest that rainbow trout can serve as a useful model species for evaluating biological responses associated with wastewater exposure under environmentally realistic conditions.