Local Salinity Conditions Affect the Feeding Ecology of Anadromous Key Fish Species in a Large European Estuary

Estuaries are both highly productive and challenging habitats for aquatic organisms due to their rapid changing environmental conditions, with salinity acting as an important driver for species composition and richness. In the Elbe estuary, one of the largest estuaries in Europe, the anadromous smelt Osmerus eperlanus is a key species and numerically dominates the fish community. Nevertheless, essential aspects of the feeding ecology and habitat use of their life stages remain unknown. Using integrated stomach content and stable isotope analyses of δ13C and δ15N, we found distinct habitat exploitation and movement patterns of juvenile and adult smelt. We observed a high overlap of shared resources with mysids and gammarids being the most important prey species. However, an ontogenetic shift was found, with isotopic overlaps between the life stages decreasing from 30.4% to 4.9% towards upstream. Adults mainly fed on prey from the mesohaline and oligohaline sections, rather than from freshwater areas. Enriched δ15N values in the maximum turbidity zone indicated starvation and a locally extended food chain which primarily affected juveniles. Our results underline the importance of estuarine habitats serving as nursery and feeding areas for different life stages of migrating fishes. The results contribute to a better understanding of habitat exploitation and intraspecific interactions of smelt along the estuarine salinity gradient and highlight the importance of estuarine services for migratory fishes.