Density, size-at-age, and distribution of young Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) in deep river pools

Abstract

A comparison of populations of juvenile Atlantic salmon (Salmo salar) and brown trout (Salmo trutta) in four deep pools (maximum depths 2.0-4.0 m) and 12 shallow riffles in three rivers showed significantly higher density, size-at-age, and biomass of the pool-dwelling salmonids. There were 2.5 times more parr per unit of area in the pools compared with the riffles (95.3 and 37.5 parr/100 m2, respectively, p = 0.0053). The pool-dwelling trout had a larger size-at-age than riffle specimens, and the pool-dwelling salmon were larger than those found in the riffles in three of four cases (p < 0.001). The relative biomass of salmonid parr in two of the investigated pools was 6.9 and 12.0 g/m2 compared with the means of 1.6 and 3.7 g/m2 in the riffles. There are two possible explanations that are discussed for the larger size-at-age of the fish in the deep pools: (i) movements of large, dominant parr into the pools as they grow older or (ii) favorable conditions in the pools that give pool-dwelling fish an energetic advantage. In contrast with established theory, the current data show that deep pools are favorable habitats for both young Atlantic salmon and brown trout.