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The alien fish species, European minnow (Phoxinus phoxinus), was recorded in the mountain lake, Øvre Heimdalsvatn, southern Norway, for the first time in 1969. After some years minnows were observed in the diet of brown trout (Salmo trutta). In 2006, brown trout had significantly higher d 15 N values than European minnow, indicating that brown trout holds the top trophic position in the lake food web. The d 15 N signatures of brown trout were significantly correlated to fish size, age and mercury concentration, while d 15 N signatures in minnows did not show any change with fish size, age and mercury concentrations, indicating that European minnow remain at the same trophic level throughout the life history. The mercury concentrations (mg kg -1 w.w.) in brown trout and minnows were in the range 0.014-0.093 and 0.014-0.16, respectively. For both species, positive relationships between total mercury concentrations and fish length, weight and age were found. Although European minnow was found as prey in brown trout stomachs, minnow had significantly higher mercury concentrations than brown trout. It is most likely that the higher mercury concentrations in the European minnow are due to the high metabolic rate, with more energy spent on maintenance than on growth as a consequence of its small size and very low annual growth rate compared to brown trout. Changes in population densities of both brown trout and European minnow may influence their habitat use, feeding preferences and metabolism, and may thus potentially lead to changes in the mercury loads of both species.

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