Survival of Endangered Snake River Sockeye Salmon Smolts from Three Idaho Lakes: Relationships with Parr Size at Release, Parr Growth Rate, Smolt Size, Discharge, and Travel Time

Abstract In 1991, Snake River sockeye salmon Oncorhynchus nerka were listed as endangered. The Sawtooth Valley Project was initiated to conserve and rebuild sockeye salmon populations that historically spawned and reared in five Sawtooth Valley lakes designated as critical habitat in central Idaho. We evaluated smolt survival of sockeye salmon that were stocked as parr into Redfish, Pettit, and Alturas lakes. Smolt travel time, residuals of Salmon River discharge and smolt travel time, and specific growth rate of parr explained 58% of the variation in smolt survival in a multiple-regression model. Smolt survival was inversely related to smolt travel time, and travel time was negatively correlated with mean May discharge in the Salmon River at Salmon, Idaho. We were particularly interested in the relationships between smolt survival and parr size at release, smolt size at migration, and parr growth rate. Smolt survival from nursery lakes to Lower Granite Dam was negatively correlated with mean parr size at release and mean smolt size at emigration. Smaller parr and smolts survived better. Smolt survival to the dam was correlated with parr growth rates in the three lakes combined; the relationship was nonlinear. Smolt survival increased as specific growth rate increased up to 0.06% per day, but further increases in growth rate were associated with reductions in survival. Absolute growth rates of parr were negatively related to parr weight at release. Smaller individuals grew faster than larger individuals, apparently as a result of size-dependent metabolic demand and prey availability. The relationships between smolt survival and parr size, smolt size, and parr growth rate differed among lakes. These data suggest that successful migrants must at least maintain weight during the winter preceding migration and that the stocking of smaller parr with lower metabolic demand may be preferable to stocking larger parr when forage is limited. Received November 18, 2010; accepted June 1, 2011