Estimating season-wide survival rates of outmigrating salmon smolt in the Snake River, Washington

Abstract

Standard release-recapture models can provide release-specific estimates of survival probabilities for a group of salmonid smolt released at a particular time and place in the river. However, reliable estimates of season-wide survival for the population of outmigrating smolt are needed in the Snake-Columbia River Basin for careful management of the resource. Alternative estimators are presented to estimate season-wide survival of spring chinook salmon (Oncorhynchus tshawytscha) smolt. Using daily fish tagging, survival for the middle 95% of the migration was estimated to be SS = 0.873 (SE = 0.005) from the tailrace of Lower Granite Dam (RK 695) to the tailrace of Little Goose Dam (RK 635) in 1995. Daily survival estimates were remarkably stable across the migration season with some evidence of decreased survival towards the very end of the migration. Sample size calculations suggest good precision can be attained (i.e., projected SE = 0.01) with tag releases as small as n = 500 fish per day (d = 7) across the outmigration. Less than daily sampling can result in season-wide survival estimates that are too imprecise for many management purposes.