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AbstractBackgroundEurasian perch (Perca fluviatilis) is an ecologically significant fish species in the Baltic Sea and has been recognized as a suitable organism to measure concentrations of mercury (Hg) contamination. The adult species occupy a high trophic position; therefore, significant levels of the hazardous substances tend to bioaccumulate in their tissues. However, the ability of the species to inhabit a wide range of feeding ground raises concerns about the adequacy of monitoring data in relation to the representativeness of measured levels of Hg at specific locations. Accounting for the migratory characteristics of this species can shed light on the origin of the analyzed specimens and thus trace Hg uptake chain. Perch samples and potential perch prey were collected at three remote stations in a fully interlinked system river–lake–coastal/transitional waters of the Gulf of Riga. Total mercury (THg) concentration and stable isotope ratios were measured in each sampled item. The perch data were divided into three subgroups associated with specific feeding grounds and one mixed group. A Bayesian mixing model was implemented to quantify the feeding preferences of each group, and based on the results, influence of each food source on Hg uptake by perch was modeled by means of Gaussian GAM model.ResultsCalculated carbon and nitrogen stable isotope values demonstrated clear evidence of perch specimens migrating between the sampling stations. Substantial proportion of specimens sampled in river and lake stations had isotopic signals consistent with feeding in the gulf. The group of perch associated with feeding in the river grounds exhibited the highest THg concentrations with mean value of 209 µg kg−1wet weight. The food itemsC. harengus membrasand Crustacean showed significant mitigating effects on THg concentration. The rest of the food items showed a secondary influence on the variation of THg concentration.ConclusionsThe study clearly showed that the high mobility of perch along associated aquatic systems has a noticeable effect on Hg concentrations measured in the fish. Therefore, trophic position and isotopic signatures, along with identification of the food sources, can serve as important supplementary tools for more accurate data interpretation of Hg accumulation.