Increasing concentrations of polychlorinated biphenyls (PCBs) in Eurasian otters (Lutra lutra) from Wales suggest...

Despite ongoing regulatory efforts to mitigate PCB pollution, their presence remains pervasive in the environment, with concentrations in top predators still reaching toxicologically significant levels. To assess temporal and spatial variation of PCB concentrations in Wales between 2010 and 2019, we analysed liver samples of a sentinel predator, the Eurasian otter (Lutra lutra), for 23 PCB congeners. PCBs were detected in all livers analysed, with PCB 153 recording the highest individual concentration (545.8 μg/kg wet weight); sixteen percent of otters had concentrations above a toxic threshold for dioxin-like PCBs. Statistical modelling revealed a negative correlation between concentrations of dioxin-like PCBs and otter body condition. While previous studies on otters from the UK showed declining PCB concentrations between 1983 and 2009, our model predictions indicated a significant increase between 2010 and 2019, mirroring trends seen in marine mammals. Higher concentrations were observed in otters from coastal, low-altitude areas. A multi-model inference approach was used to identify the best groups of predictors for each congener, suggesting that remobilisation of PCBs from riverine and coastal sediment sinks is now a major driver of PCB concentrations, with the impacts of climate change likely exacerbating remobilisation. While PCB concentrations are often below limits of detection in water, and below the current Water Framework Directive Environmental Quality Standard in fish, they remained at toxicologically relevant levels in otters from Wales. Our findings underscore the importance of biomonitoring across trophic levels, and suggest that current environmental quality standards for water and fish are not protective of top predators. Addressing environmental PCB contamination will require strengthened international efforts both to manage the significant sinks of legacy pollutants, as well as to meet climate change mitigation targets.