Juvenile steelhead movements in relation to stream habitat, population density, and body size: consequences for individual growth rates

Abstract

Anadromous salmonids can display considerable movement within their natal streams prior to outmigration, reflecting behavioural responses to biotic and abiotic conditions. Here we quantify dispersal and site fidelity in juvenile steelhead (Oncorhynchus mykiss) over four weeks in the fall, their consequences for individual growth rates, and how habitat, density, and individual body size can explain individual movement decisions. Retention rates ranged from 29% to 66% and were greater among yearlings (61%) than subyearlings (31%). Site fidelity in yearlings was best predicted by individual body mass (negative relationship), and site fidelity in both age classes was positively related to pool habitat availability. An experimental increase in density caused depressed growth rates in both age classes, and site fidelity was lower among translocated fish than among individuals originally residing in the manipulated sites. Within control sites, there were no significant consequences of movement for growth rates in either age class. These results show that movement rates can vary in response to local conditions and that growth rates in movers can equal or exceed those of sedentary individuals.