Three-dimensional microhabitat use by young pool-dwelling Atlantic salmon and brown trout

This study, conducted in deep pools in three rivers, is the first to show a clear three-dimensional habitat segregation in size groups (equivalent to age groups) of juvenile Atlantic Salmon, Salmo salar, and brown trout, S. t rutta. Young-of-the-year (YOY) held position near the river bed and the riv er bank; height above bottom and distance from river bank increased signifi cantly with fish size. Brown trout held position significantly further from the substratum, and were on average closer to the river bank, than salmon. The vertical segregation of young salmonids was most evident among young t rout, with YOY being closest to the bottom. This size-dependent segregation is probably a result of different outcomes of the trade-off between the co nflicting interests of higher food availability and greater predation risk in the upper part of the water column. We suggest that intercohort predatio n and competitive interactions were the main reasons why YOY of both specie s and salmon yearlings held positions close to the river bed. We found no e vidence of salmon and trout parr preferring; particular water depths, as st udies in shallow parts of dyers have suggested, as the correspondence of us e and availability of microhabitats at different water depths was high in t he pools. (C) 1999 The Association for the study of Animal Behaviour.