An experimental analysis of territory size in juvenile steelhead trout

Abstract

I experimentally manipulated levels of food abundance and density of competitors to determine how these factors influence the territory size of juvenile steelhead trout, Oncorhynchus mykiss. Steelhead trout were held in artificial stream channels and I followed cohorts that were fed at one of three levels of food abundance and stocked at one of three levels of fish density. By measuring territory size over a 2-month period, while the fish were growing, I was also able to assess the effects of body size in determining the size of a territory. Defended and foraging areas were similar in absolute size, but the frequency of space use was different for defence than for foraging. As predicted, territory size decreased with increasing levels of food abundance and increased with decreasing levels of fish density. In addition, territory size increased with increasing body size even after controlling the effects of food abundance and competitor density. In comparison to previous studies, territory size of steelhead trout changed more dramatically in response to changing levels of food and competitors. For territorial animals with indeterminate growth, territory size is not only adjusted as a trade-off between the costs and benefits of defence, but also with respect to body size due to increasing metabolic demands as individuals grow.