Alternative Juvenile Production Estimate (JPE) forecast approaches for Sacramento River Winter-Run Chinook Salmon

Abstract

Sacramento River winter-run Chinook Salmon are listed under the Endangered Species Act as Endangered, and there are substantial efforts to estimate, predict, and limit mortalities at various stages of their life cycle. One such effort is the annual forecast of the number of juvenile winterrun entering the Sacramento-San Joaquin Delta. The natural-origin juvenile production estimate (JPE) is defined as the number of winter-run juveniles produced from natural spawning areas that enter the Delta, and its forecast is used to determine the allowable level of winter-run incidental take at the state and federal pumping facilities located in the South Delta. Current monitoring programs in the Sacramento River basin do not allow the JPE to be directly estimated, and thus various methods have been used to forecast this value annually. Here, we describe three alternative methods for forecasting the natural-origin JPE. The methods range from the status quo approach (Method 1), which expresses the JPE forecast only as a point estimate, to two other methods that account for forecast uncertainty to various degrees. A comparison of JPE forecasts for 2018 across the three methods indicates that relative to Method 1, Methods 2 and 3 result in lower JPE forecasts, by 24% and 18%, respectively, primarily because of lower forecasts of the fry-to-smolt transition and the smolt survival rate that occurs downstream of Red Bluff Diversion Dam. Because post hoc estimates of juvenile winter-run abundance at the entrance to the Delta do not currently exist, we are unable to evaluate forecast skill among the three methods.