Effects of spawning distribution on juvenile Atlantic salmon (Salmo salar) density and growth

We test whether the spatial distribution of spawning sites in a natural population is related to the density and individual growth rate of juvenile stream-dwelling salmonids. For this purpose, fine-scale data on the spatial distribution of Atlantic salmon (Salmo salar) nest sites, juvenile densities and body size, and habitat characteristics were combined. The area of nests summed over reaches of 25 m in length was positively correlated with local young-of-the-year (YOY) densities and was the strongest predictor of local densities. In turn, increased YOY densities were related to slower growth, and YOY density was the strongest predictor of YOY size. Measured habitat variables had little effect on YOY density over the spatial scale examined, which suggests that redistribution of YOY salmon according to habitat preferences was limited. The distribution of spawning sites was therefore the best predictor of local densities and subsequently YOY growth. Thus, during the YOY stage when dispersal is limited, patchy spawning distributions combined with local density-dependence may be the dominant factor responsible for spatial variation in growth.