Demographic Responses to Food and Space Competition by Juvenile Steelhead Trout

Abstract

I conducted two experiments in artificial stream channels, manipulating density of competitors, food abundance, and the possibility of emigration, to test whether density-dependence can operate through these factors in populations of a stream-dwelling salmonid fish, juvenile steelhead trout (Oncorhynchus mykiss). In the absence of emigration, increasing levels of per capita 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 larger individuals and did so with increasing frequency as food abundance decreased 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 survivors. Emigration was more likely at increasing levels of per capita competition; emigrants were smaller and in poorer condition than nonemigrants. The ability to emigrate from a population appears to normalize final size distributions and increase mean fish size within the stream channels. Thus, although both food and space are important factors shaping the demography of stream salmonid populations, neither appears to limit salmonid abundance exclusively.