Coupling stable isotopes with bioenergetics to evaluate sources of variation in organochlorine concentrations in Baltic salmon (Salmo salar)

Atlantic salmon ( Salmo salar ) collected from three locations around the northern Baltic Sea in 2003–2004 showed large spatial and individual variation in their organochlorines (OCs) (polychlorinated dibenzo-p-dioxins and furans and polychlorinated biphenyls). This variation could be explained only partly by their size or sea age. The variability arose from the differences in salmon diet, trophic position, and prey OC concentrations and lipid content. A salmon bioenergetics accumulation model was used to evaluate the contribution of salmon growth and their diet to the observed individual variation in OC content. Our model revealed that the contribution of three main prey species in the OC accumulation of salmon varied markedly between the study areas. Amount of lipids in salmon explained a large proportion of their OC concentration. However, trophic position of salmon calculated from the δ15N values explained almost 80% of the variation in lipid-normalized OC concentrations. In the Gulf of Finland, where OC concentrations of salmon were highest, their prey species had the highest OC concentrations and trophic positions. Higher OC concentrations in the Gulf of Finland might be related to elevated trophic positions caused by invasion of the predatory cladoceran Cercopagis pengoi in 1990.