Аннотация:1. Stable isotopes of nitrogen and carbon are widely used in the evaluation and understanding of trophic interactions, and especially so in aquatic food‐webs. However, differences among tissues which occur in consumers such as fish may confound resolution of these ecological relationships, and greatly affect dietary compositions calculated through mass‐balance equations. 2. δ 13 C and δ 15 N were determined for homogenized whole fry (≈ 2·7 g) and particular tissues (white muscle, red muscle, liver and heart) of juveniles (≈ 20·6 g) of Rainbow Trout Oncorhynchus mykiss. 3. Differences in δ 13 C and δ 15 N were observed among tissues; removal of lipid resulted in these tissues becoming statistically indistinguishable but more variable in δ 13 C, particularly in tissues with higher lipid content. Differences in δ 15 N among tissues may be related to the relative composition of ‘essential’ and ‘non‐essential’ amino acids. 5. Acidification, which is used to reduce variability caused by the presence of inorganic carbonate, had a significant effect on δ 15 N, while having a non‐significant effect on the δ 13 C. This was the case for whole ground fish‐fry, which contained 22·6% inorganic carbonates in the form of bones and scales, and for individual fish tissues which contained only very small amounts of inorganic carbonate. 6. White muscle was found to be less variable in δ 13 C and δ 15 N than all other tissues, and is probably the best tissue for use in ecological work. Red muscle, which is often closely associated with white muscle, is more variable in δ 13 C and may constitute a source of significant error in source material identification and dietary overlap.
