An experimental analysis of territory size in juvenile steelhead trout

I experimentally manipulated levels of food abundance and density of compet itors to determine how these factors influence the territory size of juveni le steelhead trout, Oncorhynchus mykiss. Steelhead trout were held in artif icial 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 gl owing, I was also able to assess the effects of body size in determining th e 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 fo raging. As predicted, territory size decreased with increasing levels of fo od abundance and increased with decreasing levels of fish density. In addit ion, territory size increased with increasing body size even after controll ing the effects of food abundance and competitor density. In comparison to previous studies, territory size of steelhead trout changed more dramatical ly 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. (C) 2000 The Association for the Study of Animal Behaviour.