Effect of age-related dietary changes on mercury accumulation in European perch (Perca fluviatilis) in a large mesotrophic lake in the taiga zone (Northwestern Russia).

The European perch (Perca fluviatilis) is a fish species that can be a planktivorous, benthivorous and carnivorous at different life stages. Using the example of the perch population from White Lake (Northwest Russia) the connection between ontogenetic change of diet and mercury accumulation in fish muscle tissue is shown. Analysis of the gut contents of 223 perch indicated that the main food item of perch aged 4-6+ years is the lake form of European smelt (80-100%). Also, the lowest THg concentrations were observed in perch aged 4-5+ and 6+ years - 160 ± 47 and 172 ± 51 ng/g (wet weight), respectively. Starting from age 6+ pikeperch and ruff appear in the diet of perch. In fish aged 9+ the proportion of lake smelt in the diet is 25-33%, and the mercury content in muscles is 447 ± 44 and 462 ± 109 ng/g (wet weight) in individuals aged 9+ and 10-12+ years, respectively. An increased proportion of pikeperch and ruff in the diet, whose average mercury content is 2.5 times higher than that of smelt, lead to increased mercury concentrations in perch muscle. There was also a statistically significant linear regression between mercury content and δN in perch older than 7+ years (p = 0.002; r = 0.38) and no correlation between mercury content and δC in fish of different ages. Concentrations exceeding the U.S. EPA recommended values (330 ng/g ww) were found in perch aged 9+ years and older, with body length greater than 25 cm. Therefore, the local people should not consume perch with body length greater than 25 cm from White Lake. Further measurements are needed to determine the mercury concentration in perch in nearby lakes and their suitability for human consumption. It is recommended that a similar study be conducted in a number of reservoirs within the study region in order to determine the factors influencing the intensive accumulation of mercury in fish.