Abstract
Population-level risk assessments of oil contamination primarily focus on fish early life stage (ELS) sensitivity. However, subtle disruptions in adult reproduction may strongly influence population growth by affecting spawning stock and subsequent recruitment. We investigated the effects of crude oil water-soluble fraction (WSF) exposure during late vitellogenesis on reproductive development and spawning dynamics in adult polar cod (Boreogadus saida). Fish were exposed for 20 days to environmentally relevant, declining WSF concentrations using an oil–gravel column system. Reproductive development was monitored over 89 days using gonadosomatic index (GSI) and histological analyses. Endocrine responses were assessed through plasma sex steroids and expression of genes involved in hepatic vitellogenesis and ovarian steroidogenesis. Gamete quality (egg diameter, sperm motility and velocity, fertilization success) and spawning phenology were evaluated. Exposure caused a significant, dose-dependent advancement of female spawning timing. This phenological shift occurred without detectable effects on gonadal structure, oocyte size, egg diameter, fertilization success, or sperm motility. Subtle changes in plasma testosterone and steroidogenic gene expression suggest modulation of endocrine pathways regulating the transition from vitellogenesis to final oocyte maturation. In Arctic ecosystems, where reproductive timing is tightly coupled to seasonal productivity, contaminant-induced shifts in spawning phenology may disrupt match–mismatch dynamics and recruitment. These findings highlight the need to integrate adult stage sensitivity and phenological endpoints into population-level risk assessments of oil exposure in high-latitude fish species.