Relationships between body mass, population density, and the self-thinning rule in stream-living salmonids

In food and (or) space-limited populations, increases in average body size may be accompanied by increases in per capita space and (or) energy requirements. Thus, as body size increases in time or space, the equilibrium number of individuals that can be supported in a given environment should decrease. This relationship, referred to as self-thinning, produces a negative linear relationship between log body size (mass) and log population density. We studied body mass population density relationships in stream-living populations of several trout species from the intermountain western United States. Our analyses of relationships between body mass and population density demonstrated that self-thinning can be manifested through both temporal and spatial variability in fish population characteristics. Several populations did not show self-thinning, possibly due to species-specific traits, temporal and spatial environmental variation, or both. Thinning lines may not reveal specific causal mechanisms, but they appear to be real, density-dependent responses and thus offer a potentially useful method for studying fish populations. Experimental studies of self-thinning under more controlled conditions are needed, however, to identify causal mechanisms and eliminate alternative explanations for body size - abundance relationships.