Variability of isotopic partitioning between sympatric brown trout (Salmo trutta) and European grayling (Thymallus thymallus) in the Loue River (France)

Trophic partitioning, defining how individuals or populations differ in their resource use, is expected to promote coexistence of sympatric species by reducing resource overlap. We used stable isotope measurements (delta C-13 and delta N-15) to address niche characteristics (delta C-13 and delta N-15 ranges, isotope convex hull and standard ellipse areas, mean nearest neighbour distances and trophic positions) and trophic partitioning of sympatric brown trout (Salmo trutta, n = 110) and European grayling (Thymallus thymallus, n = 63) in two sampling locations of a French river. Aquatic resources predominantly fuelled both species, yet both terrestrial resources (TER) and trophic positions (TP) were higher for trout (similar to 36 +/- 13% TER, 3.6 +/- 0.7 TP) than grayling (similar to 26 +/- 9% TER, 2.7 +/- 0.6 TP) supporting difference in their trophic niches. Isotope analyses showed that trout had a larger isotope niche than grayling suggesting more opportunistic trophic behaviour. Their isotopic overlapping was higher at the upstream site (isotopic nestedness = 0.8 +/- 0.1) than that at the downstream site (isotopic nestedness = 0.4 +/- 0.2). Euclidian distances of stable isotopes and TP for the two species increased with salmonid size, while aquatic resource use decreased with salmonid size. These results demonstrate an increase in isotopic niche partitioning and change in trophic attributes with ontogeny. Our study showed that despite relying on similar resources, these two sympatric salmonids exhibited clear trophic differences that were amplified with ontogeny. The consideration of fish ontogenetic dietary shift would hence be a determinant driver of the trophic niche partitioning for these sympatric salmonids.

Automated summary

Both brown trout and European grayling predominantly rely on aquatic resources, but brown trout have higher terrestrial resource use and trophic positions compared to grayling. Isotope analysis reveals that trout have a larger isotope niche than grayling, suggesting more opportunistic feeding behavior. The ontogenetic shift in trophic attributes results in increased isotopic niche partitioning between the two sympatric salmonid species.