Abstract

Visibility is thought to affect the territory size of visually oriented animals but there have been few experimental tests of the hypothesis. We re-examined the relationship between visibility and territory size in juvenile salmonids to test the hypothesis that increasing habitat heterogeneity results in a reduction in territory size and consequently in higher population densities. Equal densities of young-of-the-year rainbow trout (Oncorhynchus mykiss) were stocked in two experimental treatments with low visibility and a control treatment with high visibility. Visibility was decreased by placing large stones or plywood dividers onto the substrate of experimental stream channels. As predicted, the size of individual territories decreased with decreasing visibility of the habitat. However, the treatments did not differ significantly in population density or growth rate of the fish. While this study confirms the inverse relationship between habitat visibility and territory size, the decrease in territory size did not produce an increase in population density of juvenile salmonids.