Abstract

Despite a wealth of research on territoriality and population regulation in stream dwelling salmonids, most studies have focused on manipulations of food abundance alone, or else have simultaneously manipulated space and food abundance, but used population densities far outside the range of natural densities. No experiments have tested the effects of population density on territory size and individual behaviour in salmonids over a range of realistic densities, while keeping all other factors equal. Hence, I manipulated population density of juvenile rainbow trout at five levels (two, four, eight, twelve, and sixteen fish) in artificial stream channels (1 x w = 1.47 m 2 ) while keeping per capita food supply constant. I tested the contrasting predictions that the territory size of dominant fish (1) is not affected by population density; (2) decreases with population density as 1/n; or (3) decreases with population density but towards an asymptotic minimum size. Territories of dominant fish in this experiment decreased with increasing population density before leveling off at the highest densities, and therefore seemed to support the prediction of an asymptotic minimum size, as well as the hypothesis that territoriality can regulate populations of stream salmonids. At low densities dominant fish were despotic and defended most of the available space. However, as fish abundance increased, dominants defended less space, and the social system switched from despotic to a territorial mosaic. My study has helped to clarify the role of territoriality in regulating populations of stream dwelling salmonids which may have important implications for conservation and behavioral ecology.