Relative importance of size-based competitive ability and degree of niche overlap in inter-cohort competition of Atlantic salmon (Salmo salar) juveniles

Abstract

The competitive effect of older cohorts on younger cohorts may strengthen with increasing size differences owing to increasing differences in competitive abilities. Alternatively, it may weaken owing to increasing partitioning of resources as a result of ontogenetic niche shifts. Here, we test this by creating spatial variation in densities of one size class of overyearling Atlantic salmon ( Salmo salar ) and assess the effects on two size classes of young-of-the-year (YOY). The positive relationship between growth of overyearlings and final body size of YOY (a proxy for their growth) was steeper for the larger size class of YOY than for the smaller size class, which would be expected if the degree of niche overlap between two cohorts depended on their size difference. The negative relationship between overyearling density and YOY body size was also steeper for the larger size class (at least for body mass), suggesting that effects of body size differences on relative competitive abilities appear to be of less importance than the effects on degree of niche overlap. YOY should thus experience relatively less competition from older cohorts in rapidly growing populations, and this may also apply to many other fish species with ontogenetic niche shifts.