Mercury and selenium in free-ranging brown trout (Salmo trutta) in the River Skienselva watercourse, Southern Norway.

Selenium (Se), mercury (Hg), and stable isotopes of nitrogen (δ15 N) and carbon (δ13 C) have been investigated in free-ranging brown trout (Salmo trutta) from five large lakes/hydropower reservoirs within the River Skienselva watercourse, Southern Norway. The main purpose of the study was to investigate geographical patterns of the two elements within this large catchment. We also wanted to investigate whether Hg concentration in trout were negatively associated to their Se content, hence indicating an ameliorating effect of Se on Hg bioaccumulation. Concentrations (dry weight) in trout muscle tissue ranged from 0.21 to 2.06mgHgkg-1 and 0.96 to 2.51mgSekg-1 . Covariance models revealed differences in fish Hg concentrations between lakes after adjusting for the significant contributions from both age and trophic levels (TL, measured as δ15 N), whereas fish Se concentrations differed between lakes after adjusting for TL. Se showed an inverse correlation with δ13 C signatures in trout muscle tissue, indicating increased Se uptake in pelagic feeders. Se also increased in trout in lakes towards the western part of the watercourse as well as with increasing elevation and regulation height of lakes. The inclusion of tissue Se as an explanatory variable in the Hg model was not statistically significant and increasing Se concentrations did not lead to significantly decreased mean tissue Hg concentrations in trout, after adjusting for other significant explanatory variables. Our results support previous conclusions of a muscle tissue Se concentration threshold to affect Hg concentrations in fish, and suggest that the lakes in the region most likely are too low in Se for trout to reach such a threshold concentration.