Seasonal variation of mercury and δ15N in fish from Lake Heddalsvatn, southern Norway

Seasonal variations of mercury (Hg) concentration in muscle tissue of northern pike - Esox lucius L, European whitefish - Coregonus lavaretus L and European perch - Perca fluviatilis L have been investigated in Lake Heddalsvatn, southern Norway. Our data confirmed seasonal differences in the Hg concentrations in fish, but the causes for that may vary depending on fish species. Length (pike and perch) and age (perch and whitefish) were the strongest explanatory factors for the Hg fluctuations. Fish caught in December (pike and whitefish) had the highest Hg concentrations, while the concentrations in May were higher than in September for all 3 species, statistically significant for perch (P<0.0001) and close to significant for pike (P=0.07). Higher δ15N signatures were found in December and May compared with September, but these differences might be related to fish body size variations. There was a large range of δ13C signatures in the investigated fish, but no significant differences between seasons neither within nor between species.