An experimental analysis of self‐thinning in juvenile steelhead trout

Abstract

Mobile animal populations have been proposed to decline in density according to a slope based on the allometry of metabolic requirements or space requirements. In salmonid fishes, metabolic rate and food consumption scale to body mass by the exponent 0.87 and 0.73, respectively; whereas the territory size of steelhead trout scales to body mass by the exponent 0.86. Experimental cohorts of juvenile steelhead trout (Oncorhynchus mykiss) were used to test the hypothesis that mobile animal populations composed of individuals with indeterminate growth decline in density as a result of self‐thinning. After controlling for experimentally manipulated levels of food abundance and stocking density, cohorts of steelhead trout declined in density with increasing body size according to a slope closest to the allometry of food consumption. Densities of steelhead trout were inversely related to average mass by the exponent −0.74. Despite the similarity to the food consumption slope, a relatively wide confidence interval also precluded distinguishing the slope either the metabolic rate or territory size slopes. Data from the literature were also examined to determine if there was general support for the idea of self‐thinning in natural populations of stream‐dwelling salmonid fish. Although not all data suggest that populations of salmonids in streams decline as a result of density‐dependent intraspecific competition, at least some appear to fit the idea of self‐thinning; especially when density is above a minimum level of habitat saturation.