Redistribution of juvenile salmonid fishes after localized catastrophic depletion

Abstract

Juvenile Atlantic salmon and brown trout were depleted at three sites (c. 108–380 m2) of a natural stream during the summer months of 1991 and 1992. Local population changes and movements of fish marked in sections adjacent to each depleted area were monitored thereafter. There was very little movement of marked salmon parr into the central regions of the depleted areas following the immediate post‐marking period. Upstream movement by young‐of‐the‐year fish from high density sections in mid‐late summer was noted for trout but not salmon. Unmarked 1‐year‐old salmon parr immigrated into depleted areas in June 1992, and the pattern of recolonization was consistent with migration upstream from the adjoining river. It is concluded that resident salmon were very strongly site‐attached and resource tracking was of no functional significance as a compensatory mortality mechanism. The occurrence of a long distance migratory component in the population during early‐mid summer indicates that this, rather than local resource tracking, constitutes a potential compensatory mechanism.