Demographic responses to food and space competition by juvenile steelhead trout

I conducted two experiments in artificial stream channels. manipulating den sity of competitors, food abundance, and the possibility of emigration, to test whether density-dependence can operate through these factors in popula tions of a stream-dwelling salmonid fish, juvenile steelhead trout (Oncorhy nchus mykiss). In the absence of emigration, increasing levels of per capit a food competition increased mortality, decreased growth, and increased the variance in size distributions of surviving individuals. Smaller fish were more likely to occupy less profitable areas of the stream channel than lar ger individuals and did so with increasing frequency as food abundance decr eased and stocking density increased. When I allowed fish to emigrate from the stream channels, food and stocking density again influenced mortality, growth, and size distributions of surv ivors. Emigration was more likely at increasing levels of per capita compet ition; emigrants were smaller and in poorer condition than nonemigrants. Th e ability to emigrate from a population appears to normalize final size dis tributions and increase mean fish size within the stream channels. Thus, al though both food and space are important factors shaping the demography of stream salmonid populations, neither appears to limit salmonid abundance ex clusively.