Behavioural and demographic responses to food and space competition by juvenile steelhead trout (Oncorhynchus mykiss)
Creators: name: Keeley, E.R.
Publisher: name: University of British Columbia
Year: 2009

description: In populations of plants and animals, competition for a limited amount of critical resources is often believed to lead to density-dependent changes in demographic characteristics such as population size, mortality, growth and emigration. For animals with indeterminate growth, dramatic changes in body size can mean that territory or space requirements change substantially as individuals age. Hence, increasing body size and corresponding metabolic demands will continue to cause density-dependent changes in population size. I studied the ecology of juvenile steelhead trout (Oncorhynchus mykiss) to determine how levels of per capita competition influence both demographic characteristics of trout in streams as well as their behaviour. In a field study, I observed the behaviour of juvenile steelhead trout in two rivers in British Columbia, to determine the importance of invertebrate drift abundance, intruder pressure, and body size of territory holders as correlates of territory size. Territory size increased with increasing fish size, but fish smaller than 5 cm had relatively large territories for their body size, in comparison to fish that were larger than 5 cm in length. After statistically controlling for the effect of body size, territory size was inversely related to the abundance of drifting invertebrates. Territory size was not related to the number of intrusions observed on a territory per hour, but was inversely related to local fish density, a second measure of intruder pressure. In my first experimental study, I manipulated levels of food abundance and competition by stocking different densities of steelhead trout and feeding them at one of three levels of food abundance. As predicted, I found that increasing levels of per capita food competition significantly increased mortality, decreased growth, and increased the variance in size distributions of surviving individuals. The behaviour of fish was also affected by both treatment factors. Territory size of aggressive fish was inversely related to both food and competitor density and smaller fish were more likely to occupy less profitable areas of the stream channel than larger individuals, when food abundance was low or stocking density was high. In a second experiment, I again manipulated food and stocking density, but unlike my earlier experiment, I allowed fish to emigrate from the stream channels if they chose to do so. As before, I found significant influences of both treatment factors on the mortality, growth and the shapes of size distributions of survivors. Fish were also more likely to emigrate from the stream channels, depending on the level of per capita competition and were smaller and in poorer condition in comparison to non-emigrants. The ability to emigrate appears to normalize final size distributions and increase mean fish size within the stream channels. Finally, I tested the 'self-thinning' hypothesis using the data from my two experiments for steelhead trout. I found support for the idea that stream-dwelling salmonids undergo a self-thinning process. Density of fish at the end of the experiments was inversely related to mean body size, even after statistically controlling for the effects of food and competitor abundance.