Introduced European minnow Phoxinus phoxinus in alpine lakes may increase total mercury concentration in brown trout Salmo trutta

In Norway, the cyprinid European minnow Phoxinus phoxinus has been spread far outside its previous natural distribution area, with lots of establishments in mountain lakes where brown trout Salmo trutta originally was the only fish species. We have analysed δ15N and total mercury (THg) concentration in brown trout from eight lakes, situated between 1031 and 1244 m a.s.l. on the Hardangervidda mountain plateau, southern Norway. One of the lakes is inhabited by brown trout and European minnow, while in the other seven lakes, brown trout is the only fish species. δ15N of brown trout were significantly higher in the population with co-existing European minnow, indicating a higher trophic position of brown trout in this population than in the allopatric populations, probably caused by piscivory, as indicated by frequent occurrence of European minnow in brown trout diet. The mercury concentrations in brown trout from this lake had values up to around 0.4 mg THg per kg wet weight. The concentrations were significantly higher than in the lakes without European minnow, and together with the δ15N values, indicating that translocation and establishment of European minnow may increase the trophic position of brown trout in previously allopatric populations, and thereby also increase the mercury level.