Two‐phase self‐thinning in stream‐living juveniles of lake‐migratory brown trout Salmo trutta L. Compatibility between linear and non‐linear patterns across populations?

The self‐thinning rule establishes that, in resource limited populations, increased per‐capita requirements by individuals with indeterminate growth may limit density. This invariant rule relating log‐log linear declines of cohort density with increased body mass has been well established in sessile organisms. However, few studies have assessed the occurrence of self‐thinning in mobile organisms and still fewer have demonstrated its operation in natural situations. In Bisballe Baeck, where brown trout individuals experienced the simultaneous operation of density‐dependence on growth and mortality, increased body mass explained 89.0–98.4% of the variations in density. However, relative to the invariant linear line, the decline in density with increased body mass described distinctly nonlinear, two‐phase trajectories. An initial phase of variable slopes continued until body mass attained ≈10.2 g, at which the direction of the trajectory switched to a second phase of substantially steeper slopes or severe declines of density per mass unit increment. As inferred by the highly significant relationships between these two sets of slopes and annual recruitment, the two phases were consistent with a major assumption of self‐thinning: the rate at which density declined with increased body mass increased with the intensity of intraspecific competition. The results of this study provide prima facie evidence for the occurrence of self‐thinning in a mobile species based on observational data. A further comparison among density–body mass relationships described for resident, anadromous and lake‐migratory brown trout suggests consistent two‐phase self‐thinning patterns across populations.