Habitat associations of age-0 cutthroat trout in a spring stream improved for adult salmonids

Native cutthroat trout (Oncorhynchus clarki) in the Snake River watershed use streams formed by large springs for spawning and nursery habitat. Several spring streams have been modified to enhance abundance of adult salmonids, but the habitat associations of age-0 cutthroat trout in these systems are undescribed. We assessed the frequency of collection of age-0 cutthroat trout in riffles, riffle margins, pool margins, and backwaters from late June to the middle of August 2000 in a spring stream with such modifications. The proportion of sites in which age-0 cutthroat trout were collected increased up to the middle of July and then decreased. We found substantially lower frequencies of collection of age-0 cutthroat trout in riffles compared to the three stream-margin habitat types. Age-0 cutthroat trout appeared to select shallow, low-velocity, stream-margin habitat with cover that provided protection from piscivorous adult salmonids and avian predators. Our observations suggest that modification of spring streams for production of cutthroat trout should include efforts to manage stream margins so they provide cover in the form of aquatic macrophytes or overhanging vegetation for age-0 fish.