Population-specific escapement of Columbia River fall Chinook salmon: Tradeoffs among estimation techniques

In the multi-stock Columbia River system, managers estimate fall Chinook salmon, Oncorhynchus tshawytscha (Walbaum), escapements using various combinations of spawning ground surveys, harvest data and fish counts at dams and hatcheries. Our objectives were to improve upon the traditional methods, and to evaluate trade-offs among methods. Using data from radio-tagged (n = 4421) and PIT-tagged (n = 1950) adult salmon over eight years, we applied a mark-recapture method to estimate population-specific escapements, both aggregating data within year and stratifying them by week. Mark-recapture estimates differed between estimation techniques and from estimates generated using traditional methods. Stratifying data by week measured escapement estimate uncertainty more reasonably than aggregating data within year. Radiotelemetry provided better spatial resolution among populations for tributary spawners whereas PIT tags provided low-cost, easily replicated estimates using an existing detection system. Mark-recapture techniques had several advantages over current practices: quantifying uncertainty, transparent methods and reduced sensitivity to survey biases.

► Improved techniques for spawning escapement estimation are needed for fall Chinook salmon in the Columbia River.
► A mark-recapture method was applied to data on radiotelemetry and PIT tagged salmon collected during eight years.
► Because of the variability in salmon migration timing, and sampling restricted by water temperature, we stratified data by week.
► Stratified analyses measured population-specific salmon escapement and associated uncertainty more reasonably than aggregated methods.