Identifying migratory Salmo trutta using carbon and nitrogen stable isotope ratios

Many Salmo trutta populations consist of non-anadromous (freshwater-resident) brown trout and anadromous (sea-run migratory) sea trout. Although adult brown trout and sea trout can usually be identified using differences in size and body colouration, it is not possible to easily identify eggs/alevins as the progeny of brown trout or sea trout. In this study we show that delta(13)C and delta(15)N, measured using a continuous flow isotope ratio mass spectrometer (CF-IRMS), can accurately identify fish eggs as the progeny of freshwater-resident (delta(13)C(egg) = -25.7 +/- 1.9 per thousand,delta(15)N(egg) = 9.2 +/- 1.8 per thousand) or migratory (delta(13)C(egg) = -19.9 +/- 1.1 per thousand, delta(15)N(egg) = 14. 3 +/- 1.5 per thousand) adult female Salmo trutta. Case studies show that stable isotope analysis is a more reliable technique for distinguishing anadromous adult fish than differentiation using morphological characteristics. For example, stable isotope analysis of brown trout from Loch Eck, Scotland, revealed that some individuals possessed delta(13)C and delta(15)N signatures indicative of marine feeding despite visual identification as freshwater-resident fish. It is most likely that these fish are misidentified sea trout although it possible that these fish may be brown trout that have adopted an estuarine feeding strategy to avoid interspecific competition for food within Loch Eck with salmon, powan and Arctic charr. Most stable isotope studies of fish ecology use terminal tissue sampling to provide sufficient biological material for isotopic analysis; however, our study suggests that adipose fin tissue could provide a comparable measure of delta(13)C and delta(15)N. Such a strategy would be invaluable when studying the trophic ecology or migration patterns of fish of high conservation value.