Differentiation of Parasite Communities in Juveniles of Atlantic Salmon and Brown Trout as an Indicator of Their Population Segregation in Three Scottish Rivers

ABSTRACT Interactive segregation is considered to be the main mechanism for reducing competition, limiting the size and overlap of niches when juvenile Atlantic salmon ( Salmo salar Linnaeus, 1758) and brown trout ( Salmo trutta Linnaeus, 1758) co‐occur. However, the differences observed between allopatric and sympatric populations of these fishes in their habitat utilisation are not always evident. We propose to use degree of the parasite communities divergence as an integral indicator of segregation, as it is known that similar infection is found in fish with similar behaviour and habitat use and vice versa. The parasite communities were studied in juvenile Atlantic salmon ( Salmo salar Linnaeus, 1758) and brown trout ( Salmo trutta Linnaeus, 1758) inhabiting the rivers South Esk and Spey (north‐eastern Scotland, flowing into the North Sea) and the Annan (south‐western Scotland, flowing into the Irish Sea). The parasitological data were used to test the interactive segregation hypothesis, which is thought to be the main mechanism of avoiding competition and determining the degree of niche overlap between sympatric fish species. In all the rivers, parasite communities of brown trout were richer than those of salmon. Ordination of infracommunities using non‐metric multidimensional scaling (NMDS) indicated the greatest and the smallest overlap between parasite communities of salmon and brown trout in the South Esk and the Spey, respectively. The analysis of similarity (ANOSIM) confirmed that intraspecific differences in the parasite communities of salmon and brown trout were consistently lower than interspecific differences, increasing from the South Esk via the Annan to the Spey. The trend of increasing segregation of the host populations was supported by NMDS‐coordination of the component communities of the parasites based on parasite prevalence, comparison of lists of core species and species accumulation curves. Accumulation models indicated the lowest expected species richness of infracommunities in both hosts in the South Esk (a small mountain river), and the highest expected species richness in the Spey (the largest of the studied rivers with a high habitat diversity). In the Annan, a southern river flowing in the plains but similar to the South Esk in length and catchment area, the parasite infracommunities of salmon were as poor as in the South Esk, but those of brown trout were as rich as in the Spey. This differentiation is presumably associated with a high trophicity of the Annan and thus an improved food resources for fish. Juvenile brown trout uses these food resources more actively than juvenile salmon, which is more narrowly specialised and prefers habitats with fast‐flowing water. A smaller divergence of the model curves in the Annan as compared to the Spey, indicating a lesser segregation of the host populations, can be explained by a much higher rate of parasite species accumulation in salmon. The potential of using fish parasites as biological indicators in freshwater ecosystems is discussed.