Stable isotope evidence of trophic interactions between introduced brook trout Salvelinus fontinalis and native brown trout Salmo trutta in a mountain stream of south‐west France

The potential trophic impact of introduced brook trout Salvelinus fontinalis on native brown trout Salmo trutta in a mountain stream (south‐west France) was investigated using stable isotope analysis (SIA). The isotopic signatures (δ 13 C and δ 15 N) of S. fontinalis were similar regardless of the absence or presence of S. trutta , and SIA mixing models revealed that S. fontinalis diet consisted mainly of terrestrial invertebrates. Conversely, a significant shift in S. trutta isotopic signatures (depletion of 1·6‰δ 13 C and enrichment of 0·6‰δ 15 N) was observed in sympatry with S. fontinalis ; this may be due to a dietary shift towards terrestrial invertebrates. Contrary to an expected dietary divergence in sympatry, an elevated level of dietary overlap was observed between the non‐native and native salmonids when in co‐occurrence. This dietary convergence is more likely to be due to behavioural interactions than to variations in food availability or fish displacements.