Fish fins and scales as non-lethally sampled tissues for stable isotope analysis in five fish species of north- eastern Spain

White muscle has been traditionally used in determining δ 13C and δ 15N signatures in fish but ethics and the current conservation status of many species mean the application of non-lethal sampling procedures is mandatory. In this study we test whether fins and scales can be used as non-lethal alternatives of muscle tissue for stable isotope analysis by comparing their δ 13C and δ 15N signatures, in two native (Barbus haasi, Squalius laietanus) and three introduced fish species (Rutilus rutilus, Lepomis gibbosus and Alburnus alburnus), collected in rivers from north-eastern Spain. Our results showed fins and scales were generally enriched in 13C and depleted in 15 N compared to muscle, and that both tissues were generally a moderate predictor of the isotopic composition of muscle for most species with the exception of Ebro chub (R 2 = 0.59–0.97). This study highlights the importance of determining the isotopic relationship between lethal and non-lethally sampled tissues before making straightforward equivalences, and also pinpoints the need for a more comprehensive understanding of how isotopic discrimination occurs in fins, scales and muscle tissues