Factors Affecting Survival of Subyearling Chinook Salmon at Little Goose Dam in 2013

In 2012 and 2013, PNNL estimated the dam passage survival (S_Dam) of subyearling Chinook salmon (CH0) at Little Goose Dam (LGS).  Although S_Dam exceeded the BiOp performance standard in 2012, the standard was not met in 2013.  Therefore, a study was initiated to identify the factors that influenced S_Dam in 2013 and potential alternative dam operations that optimize S_Dam of CH0 for use in future years. 

Multivariable logistic modeling results indicated discharge, water temperature, and tailrace egress time had the greatest influence on CH0 survival.  CH0 that passed LGS when discharge was <50 kcfs (mean temp.=16.9^oC) had a single release survival probability (S₁) of 0.83¹ and median tailrace egress time of 57 min compared to S₁=0.92 and 37 min for those that passed when discharge was >50 kcfs (mean temp.=15.2^oC).  We identified an operation that was used briefly in 2013 at discharges <50 kcfs during which 73 CH0 passed LGS with S₁=0.92 and median tailrace egress time of 40 min.  This information may be used to alter operations during future periods of low discharge to optimize S_Dam of CH0 at LGS.   

¹Using the virtual paired-release model, S₁ ≥0.88 was required in 2013 to achieve the CH0 BiOp standard of S_Dam=0.93.