Predation of Naturally-Produced Subyearling Chinook by Hatchery Steelhead Juveniles in Western Washington Rivers

There is considerable uncertainty about the risk of hatchery steelhead predation on wild subyearling Chinook salmon and this is a particular concern in areas containing ESA-listed populations. Between 2003 and 2005 we studied juvenile steelhead predation on Chinook salmon fry using stomach content analysis. Juvenile hatchery-origin steelhead trout were released in the Deschutes, Green, Coweeman and Kalama rivers upstream of and within known fall Chinook rearing areas in western Washington. In all years, actively migrating steelhead smolts were captured in rotary screw traps and stomach contents were checked by gastric lavage or dissection. In 2003 and 2004, non-migratory steelhead were captured by angling and electrofishing in the Deschutes River and gut contents were inspected. Salmonid fry or parts thereof in the gut were identified to species. We attempted to compare the incidence of predation on Chinook fry by migrating hatchery steelhead under different release strategies including (1) high in a watershed vs. low, (2) early release vs. late, (3) release from acclimation ponds vs. direct plants and (4) release of local, native stock vs. domesticated stock. The actual incidence of predation by hatchery steelhead on fall Chinook was uniformly low across all release scenarios tested. Of 6,029 hatchery steelhead examined, 10 fall Chinook fry had recently been consumed (0.002 fry/stomach). The range of observed predation across the various release groups of hatchery steelhead was 0 and 0.01 fry/steelhead stomach with considerable variation in the incidence of predation between streams and years. The low incidence of predation precluded statistically valid inferences related to the effect of different release strategies on predation rates. We did show that steelhead release protocols used widely in the Pacific Northwest were associated with negligible predation by migrating hatchery steelhead on fall Chinook fry. The data on predation by non-migratory steelhead juveniles were limited but support the same conclusion. The low incidences of predation observed may be a result of the timing of hatchery steelhead releases. Most subyearling Chinook salmon had already emigrated or had grown large enough to reduce or eliminate their susceptibility to predation when hatchery steelhead entered the rivers.