Identifying migratory Salmo trutta using carbon and nitrogen stable isotope ratios

Many Salmo trutta populations consist of non-anadromous (freshwater-residen t) brown trout and anadromous (sea-run migratory) sea trout. Although adult brown trout and sea trout can usually be identified using differences in s ize and body colouration, it is not possible to easily identify eggs/alevin s as the progeny of brown trout or sea trout. In this study we show that de lta(13)C and delta(15)N, measured using a continuous how isotope ratio mass spectrometer (CF-IRMS), can accurately identify fish eggs as the progeny o f freshwater-resident (delta(13)C(egg) = -25.7 +/- 1.9 parts per thousand, delta(15)N(egg) = 9.2 +/- 1.8 parts per thousand) or migratory (delta(13)C( egg) = -19.9 +/- 1.1 parts per thousand, delta(15)N(egg)= 14.3 +/- 1.5 part s per thousand) adult female Salmo trutta, Case studies show that stable is otope analysis is a more reliable technique for distinguishing anadromous a dult fish than differentiation using morphological characteristics. For exa mple, stable isotope analysis of brown trout from Loch Eck, Scotland, revea led that some individuals possessed delta(13)C and delta(15)N signatures in dicative of marine feeding despite visual identification as freshwater-resi dent fish, It is most likely that these fish are misidentified sea trout al though it possible that these fish mag be brown trout that have adopted an estuarine feeding strategy to avoid interspecific competition for food with in Loch Eck with salmon, powan and Arctic charr. Most stable isotope studie s of fish ecology use terminal tissue sampling to provide sufficient biolog ical material for isotopic analysis; however, our study suggests that adipo se 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 o r migration patterns of fish of high conservation value. Copyright (C) 2000 John Wiley &; Sons, Ltd.