Abstract

Self-thinning is a progressive decline in population density caused by competitively induced losses in a cohort of growing individuals and is frequently depicted as: $\log\sb{10}$ (density) = $\rm c -\beta\log\sb{10}$ (body mass). In mobile animals, two self-thinning mechanisms have been proposed. The territory-size hypothesis predicts that maximum population density for a given body size is set by the allometry of territory size. The energetic equivalence hypothesis suggests that if there is a constant amount of energy flowing through a population, the self-thinning slope is set by the allometry of metabolic rate. I tested the predictions of both hypotheses by monitoring body size, food availability, habitat features and population density for young-of-the-year Atlantic salmon (Salmo salar) in Catamaran Brook, New Brunswick, throughout their first growing season. In general, the results were consistent with the predictions of the territory-size hypothesis since the observed densities were high enough to expect competition for space while they did not exceed the maximum densities predicted. Moreover, the observed self-thinning slope of $-$1.16 was consistent with the slope predicted by the allometry of territory size established specifically for the population under study. The overall slope, however, was steeper than predicted by the allometry of metabolic rate, most likely because of a gradual decline in food abundance over the study period. The results suggest that territoriality may play a role as a mechanism of self-thinning in populations of indeterminate growers competing for food and space.