Habitat quality enhances spatial variation in the self-thinning patterns of stream-resident brown trout (Salmo trutta)

This study explored the extent to which variation in habitat factors related to growth and density influence self-thinning patterns in stream-living brown trout (Salmo trutta). Analysis of 110 cohorts at 12 sites of four contrasting streams revealed density-mass relationships in two phases. Density of survivors decreased little during the first half of their lifetime. A second phase commenced as individuals attained a threshold mass upon which density declined linearly with increased mass. The slopes of the second phase were greater than predicted by space and food demands. Among sites, these slopes were related to threshold densities at the beginning of the second phase. In turn elevations threshold densities, and slopes depicted concave trajectories against site depth whereas threshold masses increased linearly. Apparently, cohorts remain below the carrying capacity during the first half of their lifetime and self-thin during the second half. Space-limited habitats impose site-specific carrying capacities and site-specific self-thinning coefficients, suggesting a common mechanism underlying self-thinning and an unanticipated, emerging property: two-phase patterns with far more variation in self-thinning coefficients. Variability in growth and density exhibited by brown trout and other salmonids across regions suggests that two-phase patterns may occur broadly, and self-thinning coefficients may vary widely.