The feeding ecology of pink salmon juveniles in northern Norwegian rivers and their role as prey to native salmonids

During spring and early summer 2022, ten rivers in northern Norway were sampled for juvenile pink salmon. A few of these rivers were sampled twice. The pink salmon developmental stage supported predictions from fry emergence time modelling based on river temperature and spawning in mid-August. Predicted fry emergence timing corresponded with the rise of water temperatures in spring. Nearly all salmon fry containing visible yolk sac remnants had empty stomachs, whereas the percentage of fry with stomach content increased with increasing size for individuals without yolk sac remnants. This pattern was accompanied by decreasing δ13C and δ15N values with increasing body length, indicating an ontogenetic niche shift from marine to freshwater energy resources. Chironomids constituted approximately 60% of the diet, the remaining diet parts mostly other insect groups. Individuals feeding on pink salmon fry were observed both among sampled juvenile Atlantic salmon and brown trout. Our results shows that pink salmon may survive and be ready for smolt migration in a wide range of northern Norway rivers, that pink salmon juveniles are potential competitors for food, as well that pink salmon may be potential prey items for native salmonids. The strength of these interactions will follow the amount of pink salmon juveniles that remain in the river to start feeding, which largely depends on the pink salmon smolt migration timing and pattern. This is a field where we have very little knowledge and data from northern Norway.