A Bioenergetic- and Habitat-Based Examination of Self-Thinning Relationships in Juvenile Steelhead Across a Stream Network

The self thinning hypothesis states that as individuals grow they require a larger fraction of the available resources, and this competition regulates population densities. Experiments in steelhead showed that competition caused increased mortality, lower and more variable growth rates, and increased emigration of smaller fish. Across a stream network the carrying capacity varies temporally and spatially from both a productivity and bioenergetic perspective, so in order to understand the mechanisms at action one must account for these differences. Here, we assess the prevalence and strength of the self-thinning curves in a population of wild steelhead in North-Central Idaho, USA, and relate the patterns to variation in habitat and bioenergetic conditions.

Overall, fish allocated considerable amounts of their consumed energy towards maintenance metabolism when temperatures were high. These proportions were dynamic over time, suggesting food limitation during baseflow conditions in summer and early fall, and variable over the stream network as a function of population densities and productivity. Self thinning patterns were not apparent in sites with low densities and under conditions where fish had relatively lower metabolic costs. The steepness of the self-thinning curves depended largely on the distribution of energetically favorable conditions and densities at the onset of baseflow.