Intraspecific food resource partitioning in Atlantic salmon ( Salmo salar) parr in a subarctic river

Abstract

The food utilisation and partitioning between three age-groups of Atlantic salmon parr were studied in the subarctic river Tana (70° N, 27° E) by analysis of fish stomach contents and invertebrate composition in the three main lotic feeding habitats: bottom substratum, water column and surface. The salmon parr exhibited large seasonal variation in their food choice, but the different age-groups had a similar diet, dominated by mayfly and stonefly nymphs in May, flying insects and simuliid larvae and pupae in July, and caddis fly larvae in August and September. Some differences in the food resource use of the three age-groups were however also observed, mainly related to size-dependent differences in feeding abilities, but partly also to different use of feeding habitats. Small-sized stonefly and mayfly nymphs and simuliid larvae and pupae decreased in importance with increasing fish age, whereas Trichoptera larvae and flying insects increased. The low resource partitioning between the different age-groups of salmon parr suggests that intraspecific competition for food may occur. All age-groups mostly fed on prey types associated with the bottom habitat, and substantial drift feeding was only observed in July, probably due to low drift rates throughout most of the summer season in this subarctic river.