Intracohort and intercohort spatial density dependence in juvenile brown trout (Salmo trutta)

An increased mobility with age can be expected for many organisms, which will reduce the potential for spatial density dependence. Here we quantify the extent of spatial density dependence for two juvenile age classes of brown trout ( Salmo trutta ) that differ in their dispersal abilities. As predicted, spatial variation in intracohort density had a strong effect on the performance of underyearlings, but not on yearlings. However, rather surprisingly, local underyearling density influenced the energetics of the older age class to the same extent as their own. Thus, yearlings do not appear to prioritize growth rate per se as a cue in movement decisions. The spatial patterns of densities and performance are consistent with older fish distributing themselves primarily according to preferred abiotic variables that also influence energetics. Such decisions would remove any relation between local density of their own age class and growth performance. However, because of competitive effects from the patchily distributed younger age class, spatial homogenization of competition intensities appears to be constrained. Variation in yearling body mass also increased with density of their own age class, indicating that even though the average effect of intracohort competition was absent, the individual response was asymmetric.