Stable isotope evidence of trophic interactions between introduced brook troutSalvelinus fontinalisand native brown troutSalmo truttain a mountain stream of south‐west France

Abstract

The potential trophic impact of introduced brook troutSalvelinus fontinalison native brown troutSalmo truttain a mountain stream (south‐west France) was investigated using stable isotope analysis (SIA). The isotopic signatures (δ13C and δ15N) ofS. fontinaliswere similar regardless of the absence or presence ofS. trutta, and SIA mixing models revealed thatS. fontinalisdiet consisted mainly of terrestrial invertebrates. Conversely, a significant shift inS. truttaisotopic signatures (depletion of 1·6‰δ13C and enrichment of 0·6‰δ15N) was observed in sympatry withS. 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.