Effects of Temporary Tributary Use on Escapement Estimates of Adult Fall Chinook Salmon in the Deschutes River, Oregon

International managers use escapement estimates of the Deschutes River, Oregon, population of fall Chinook salmon Oncorhynchus tshawytscha to forecast abundance and assess population health. Fish are externally marked in the Deschutes River, and a subsample of the marked fish is recovered on the spawning grounds to provide data for estimating escapement. Escapement would be overestimated if some of the marked fish exited the Deschutes River prior to spawning (i.e., used this tributary temporarily). We used mark-recapture and radiotelemetry techniques to calculate (1) the proportion of adult fall Chinook salmon that entered the Deschutes River and subsequently exited the river prior to spawning and (2) the effect of such temporary entrances on spawning ground escapement estimates. We used separate criteria to calculate maximum and minimum temporary tributary use rates, which were then used to adjust the escapement estimates made with external-tag data alone. Over the 3 years of study, the adjusted estimates were approximately 4-29% lower than the unadjusted estimates. We conclude that failure to adjust escapement estimates for temporary tributary use might lead to inflated harvest targets. To partially account for temporary tributary use, the lower 95% confidence limit for an escapement estimate could be used to regulate harvest.