PESTICIDE BIOACCUMULATION IN FISH FROM AN AGRICULTURAL WATERSHED: PATTERNS, DRIVERS, AND IMPLICATIONS FOR MONITORING.

Pesticide occurrence, composition, and bioaccumulation patterns in fish from the San Salvador River watershed (Uruguay) were evaluated by integrating biological, chemical, and environmental data. A total of 158 fish samples representing 31 species were analyzed across four seasons at nine sampling sites spanning a longitudinal gradient from headwaters to the river mouth, and results were compared with pesticide occurrence in water. Twenty-nine pesticide compounds were detected in fish tissues, with insecticides, particularly pyrethroids and organophosphorus compounds, the most abundant. Metolachlor, bifenthrin, and chlorpyrifos were the most frequently detected and most concentrated pesticides. Spatially, pesticide accumulation increased toward downstream sectors, while seasonal differences were moderate, with greater compound diversity in spring. Water and fish exhibited statistically distinct and complementary pesticide profiles: polar herbicides predominated in water, whereas hydrophobic insecticides and fungicides were frequently detected only in fish tissues. Multivariate analyses confirmed a strong separation between matrices, demonstrating that fish integrate long-term and sediment-associated exposure pathways often overlooked by water monitoring alone. Pesticide accumulation was significantly associated with compound hydrophobicity and fish lipid content, although these factors explained only a limited proportion of the observed variation, whereas no significant association with trophic position was detected. Some omnivorous and benthic-associated species exhibited relatively high pesticide concentrations or compound diversity, suggesting that habitat-related exposure pathways may also contribute to interspecific differences. Rarefaction analyses provided a robust approach for comparing pesticide diversity among species while accounting for sampling effort, revealing complementary biomonitoring roles. Overall, these findings support multi-species fish biomonitoring as a valuable tool for pesticide management and environmental surveillance in agricultural watersheds.