First feeding diet of young brown trout fry in a temperate area: disentangling constraints and food selection

Abstract

Diet composition of newly emerged brown trout fry in natural areas remains poorly known, and foraging abilities at this early stage, although presumably reduced, are still under discussion. We have studied gut content composition of brown trout fry in a temperate area (Galicia, NW Spain) and compared it to the benthic macroinvertebrate community. Small prey such as chironomid larvae and baetid nymphs were the most important food items for newborns, some of them still presenting yolk remnants. However, the positive selection observed for Polycentropodidae and Simuliidae and the rejection of Elmidae and Leuctricidae suggest that other factors apart from size, such as locomotor abilities of fish or accessibility and antipredator behaviour of prey play an important role in feeding behaviour. Additionally, analysis of diet changes on the studied fry suggests a dramatic shift in niche breadth at the moment of complete yolk absorption, which might be related to the improvement of swimming and handling ability of fry for capturing and ingesting prey. The presence of aerial imagoes only in the stomachs of fry with no yolk provides further support to this hypothesis. Planning of restoration works on spawning grounds should then allow enough time for complete recolonization by benthic macroinvertebrates, including first instars, as searching for food in newborns is limited to the nest area due to mobility constraints.