Individual space use strategies of wild juvenile Atlantic salmon

Movements of 60 stream-dwelling wild Atlantic salmon Salmo salar (97-118 mm), each tagged with a passive integrated transponder, were monitored during Four trials in an enclosed section (24 m long, 45.1 m(2) total area) of a stream at a range of densities (four, eight, 16 and 32 fish per enclosure). patterns of space use differed markedly between individuals, with 80% of fish establishing home ranges within 8 days of introduction to the enclosure (settlers) and the remainder continuing to move throughout the length of the enclosure (non-settlers). Although aggressive interactions were quite frequent and dominant fish were observed chasing subordinates, there was considerable overlap of home ranges of settlers at all densities; this was the case even at lower densities at which only a fraction of the enclosure was used by the fish. Thus, rather than adopting fixed territories, the salmon showed a high Level of space sharing. Individual fish used the same local area in different ways, ranging from highly localized Feeding on drifting food items to a wider-ranging strategy of specialising on benthic food. Among the fish that settled absolute growth rates were inversely related to body size, and at high densities fish lost weight. These results suggest that space use in wild juvenile salmon is more complex than a mosaic of territories, that salmon demonstrate significant variability in individual space use patterns, and that large fish may suffer disproportionately when populations are at the carrying capacity of their environment.