Spatial ontogeny of lentic age-0 rainbow trout, Oncorhynchus mykiss:: whole-lake manipulations of population size structure

We assessed the influence of the presence of larger-bodied conspecifics on the spatial ontogeny of age-0 rainbow trout, Oncorhynchus mykiss. We created populations with three different size structures in experimental lakes: age-0 and larger-bodied conspecifics, age-0 only, and an initially age-0 only population into which we added larger-bodied conspecifics midexperiment. In the continuous presence of larger conspecifics, age-0 rainbow trout spatial distribution was skewed toward littoral and benthic habitats which had low abundances of their primary prey (zooplankton). The opposite pattern was observed in the experimental lake devoid of larger-bodied conspecifics where the largest proportion of the age-0 class was found in the open-water limnetic habitat where their prey was most abundant. The change in spatial distribution of age-0 rainbow trout in the lake with a midseason addition of larger-bodied conspecifics was consistent with a treatment effect. This addition effect was weakened by a significant ontogenetic change in spatial distribution from littoral to benthic to pelagic habitats as the age-0 rainbow trout grew, and the strength of the change was inversely related to the intensity of interactions with conspecifics. This study provides experimental evidence that the risk of interactions with larger-bodied conspecifics is the process that controls the ontogeny of habitat use by young fish in lakes.