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Competition between brown trout and Atlantic salmon parr over pool refuges during rapid dewatering

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Variations in distributions and behaviours of Atlantic salmon Salmo salar in allopatry (homogeneous) and in sympatry with brown trout Salmo trutta (mixed) were observed before, during and after 2 day periods of dewatering in a large glass-sided indoor stream at densities typical of Scottish upland streams. Brown trout utilized pools more than Atlantic salmon at normal flows and in both species the majority of fishes moved into pools during dewatering. There was no significant effect of brown trout, which was the more dominant species, on the overall ability of Atlantic salmon to use pool habitat as a refuge during dewatering. Within mixed and homogeneous groups, average feeding levels decreased during dewatering. The highest ranking fish, which was always a brown trout in mixed groups, predominantly monopolized the pool and other individuals in pools adopted a more cryptic, stationary behaviour. Dewatering effectively increased local population density with the result that dominance status became much more important in maintaining food intake, and polarization between the top ranking fish and others increased. During the first day of dewatering, there was extreme behavioural polarization such that the dominant fish exhibited most aggression and least feeding within the group. Among dominant fish on the second day of dewatering, aggression had largely abated and feeding had returned to pretreatment levels despite the reduced average feeding within the group. The main difference between mixed and homogeneous groups was in the behaviour of the most dominant Atlantic salmon, which was near-despotic in allopatry and subordinate to brown trout in sympatry.
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Keywords: Salmo salar; Salmo trutta; behaviour; competition; dewatering; refuges

Document Type: Regular Paper

Affiliations: 1: Fisheries Research Services, Freshwater Laboratory, Faskally, Pitlochry, Perthshire, PH16 5LB, Scotland, U.K. 2: Cardiff School of Biosciences, Cardiff University, Main Building Park Place, P. O. Box 915, Cardiff, CF10 3TL, Wales, U.K. 3: School of Biological and Environmental Sciences, University of Stirling, Stirling, FK9 4LA, Scotland, U.K.

Publication date: 01 March 2008

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