Evaluation of the Dietary Composition of Eurasian Perch (Perca Fluviatilis) in an Interconnected River-lake-gulf Aquatic System as a Supplementary Tool for the Interpretation of Measured Mercury (Hg) Concentrations (One-Year Study).
Abstract BackgroundEurasian Perch (Perca fluviatilis) is one of ecologically significant fish species in the Baltic Sea and has been recognized as a suitable organism to measure concentrations of hazardous substances that characterize levels of local pollution (e.g. heavy metals or persistent organic pollutants). However, the ability of the species to inhabit a wide range of feeding grounds raises concerns about the adequacy of monitoring data in relation to the representativeness of measured levels of hazardous substances at specific locations. Accounting of the migratory characteristics of this species can shed light on the origin of the analyzed specimens and thus trace the pollution uptake chain. Perch samples and potential perch prey were collected at three remote stations in a fully interlinked system river– lake–coastal waters of the Gulf of Riga. Mercury (Hg) concentration and stable isotope ratios (13C/12C and 15N/14N) 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 modelled by means of Gaussian GAM model.ResultsCalculated stable isotope ratios of δ13C and δ15N demonstrated clear evidence that perch specimens migrate 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 river grounds exhibited the highest Hg concentrations with mean value of 209 µg kg-1 wet weight. The food item C. harengus membras and Crustacea showed significant mitigating effect on Hg concentration. The rest of the food items were not significant at the α = 0.05 level, although had different directions of the influence and slope values.ConclusionsThe study clearly showed that the high mobility of perch along associated aquatic systems has noticeable effect. 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.