Nonlinear self-thinning in a stream-resident population of brown trout (Salmo trutta)

Self-thinning, the reduction in abundance of a cohort of growing organisms due to competition for limiting resources, has been observed in mobile animals, including many stream-dwelling salmonid fishes. Identifying self-thinning and hypothesizing its causal mechanisms rely on inspection of the slope of the relationship between log body mass and log density. We have examined such relationships for 42 brown trout cohorts (11 yr classes at four sites) sampled bimonthly throughout their complete life span. Trajectories displayed two distinct phases whose transition occurred at a body mass of 30-40 g (14-15 cm). Split lines provided a significantly better fit than straight lines in 32 of 42 cases and explained an average of 81.3% of the variation in trout abundance. The slopes of the first phase typically were not significantly different from zero and were shallower than predictions from either energetic or territorial allometry, whereas those of the second phase were steeper. This pattern strongly suggested that resource availability for brown trout was size-dependent in our study sites. Cohorts did not appear to be self-thinning initially, but resources (presumably pool habitat) seemed to become progressively more limiting as trout grew to lengths exceeding 14-15 cm. Thus, larger size may allow mobile animals to exploit previously inaccessible resources but also may render previously used resources unavailable. Therefore, the relationship between per capita resource availability and body size is likely to depend on local conditions and may not be directly predictable only from scaling the needed amount of food or space with body size.