Consistent patterns in trophic partitioning between sympatric salmonid fishes in two rivers of contrasting productivity

In environments with limited prey resources, coexisting and morphologically similar species that share these resources can compete strongly, potentially resulting in competition and trophic niche displacement. Alternatively, they can partition in their resource use to minimise their competitive interactions. Here, the trophic relationships of two sympatric salmonid fishes, brown trout Salmo trutta and Atlantic salmon Salmo salar, were assessed in two contrasting rivers, a chalk stream where the fish were very fast growing, and an upland stream where the fish were relatively slow growing. Using stable isotope analysis (δ13C and δ15N), the size and position of their stable isotope niches were assessed when the species were sympatric and, in the upland stream, compared between allopatry and sympatry. In both rivers and all sympatric sites, strong patterns of inter-specific stable isotope niche partitioning were evident. In both species in the upland stream, there were only minor differences between their isotope data between allopatry and sympatry, with the position of their isotopic niche similar in both contexts and with overlap in the 95% credible intervals of their isotopic niche sizes. This suggests inter-specific differences in their trophic ecology were driven by differences in functional morphology and habitat use than inter-specific interactions.