| Summary: | Understanding drivers of mercury (Hg) accumulation in freshwater food webs is vital due to its ability to accumulate in organisms and magnify in food webs to potent levels, potentially impacting wildlife and human health. This is of particular concern in regions where fish serve as an important food source. This study explores drivers of Hg accumulation in European perch (Perca fluviatilis) across two subarctic lakes, Skrukkebukta and Vaggatem, within the Pasvik watercourse in northern Norway, focusing on the influence of ontogenetic dietary shifts and lake-specific ecological factors. To assess main predictors for total Hg (THg) in perch muscle tissue, this study combines data on stomach contents (n=892 fish), stable isotopes of nitrogen and carbon (δ15N, δ13C; n=179) and THg (n=385) for fish collected between 2018–2021. Perch in Skrukkebukta had higher THg levels than those from Vaggatem, potentially reflecting the proximity of Skrukkebukta to historical local pollution sources, but also seemingly related to lake-specific factors such as food web structure, prey availability and habitat features. Fish length, age, and trophic position (indicated by δ15N) were the primary predictors of accumulation of THg. Larger, older individuals had higher THg, aligning with accumulation over their lifetime and their piscivorous diet at later ontogenetic stages. Additionally, sex was found to be a significant factor in predicting variation in THg, suggesting that physiological differences between male and female perch may cause differences in THg concentrations. This study supports that ontogenetic dietary shifts strongly impact the THg accumulation in perch, but also demonstrates that other ecological and life-history factors can be contributors. The findings emphasize the importance of considering species-specific life history traits and dietary factors in freshwater ecosystem management and assessment of public health risks associated with Hg exposure through fish consumption.
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