Ontogenetic Niche Shifts and Life‐History Variation Shape Mercury Accumulation in European Perch

Mercury (Hg) bioaccumulates in aquatic organisms and biomagnifies through food webs, potentially reaching concentrations that affect fish, wildlife and human health. Because Hg concentrations generally increase with trophic position, ontogenetic shifts towards piscivory can increase dietary Hg exposure as fish begin consuming prey with higher Hg concentrations than lower‐trophic invertebrates. Here, we investigated drivers of Hg accumulation in European perch ( Perca fluviatilis ) from two subarctic lakes by combining stomach‐content data, carbon and nitrogen stable isotopes, and total mercury (THg) concentrations. Fish length and trophic position (indicated by δ 15 N) were the strongest predictors of THg concentrations, and length‐based models performed substantially better than age‐based models. Increasing fish size was accompanied by an ontogenetic dietary shift from predominantly invertebrate prey towards piscivory. Mean THg concentrations were higher and the relationship between THg and δ 15 N was steeper in lake Skrukkebukta than in lake Vaggatem. Males had higher THg concentrations than females after accounting for other predictors, while body condition was negatively associated with THg. Overall, our results indicate that ontogenetic changes in trophic ecology are important drivers of Hg accumulation in perch, while sex, body condition and lake‐specific conditions contribute additional variation. The findings highlight the importance of considering species‐specific trophic ecology and life‐history variation, particularly differences in somatic growth, when assessing Hg dynamics and fish‐consumption risks in freshwater ecosystems.