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 create d populations with three different size structures in experimental lakes: a ge-0 and larger-bodied conspecifics, age-0 only, and an initially age-0 onl y population into which we added larger-bodied conspecifics midexperiment. In the continuous presence of larger conspecifics, age-0 rainbow trout spat ial distribution was skewed toward littoral and benthic habitats which had low abundances of their primary prey (zooplankton). The opposite pattern wa s observed in the experimental lake devoid of larger-bodied conspecifics wh ere the largest proportion of the age-0 class was found in the open-water l imnetic habitat where their prey was most abundant. The change in spatial d istribution of age-0 rainbow trout in the lake with a midseason addition of larger-bodied conspecifics was consistent with a treatment effect. This ad dition effect was weakened by a significant ontogenetic change in spatial d istribution from littoral to benthic to pelagic habitats as the age-0 rainb ow trout grew, and the strength of the change was inversely related to the intensity of interactions with conspecifics. This study provides experiment al evidence that the risk of interactions with larger-bodied conspecifics i s the process that controls the ontogeny of habitat use by young fish in la kes.