Estimates of Adult Fall Chinook Salmon Escapement in Lower Columbia River Tributaries, 1943-2009

The Washington Department of Fish and Wildlife (WDFW), formerly the Washington Department of Fisheries, has monitored fall Chinook salmon (Oncorhynchus tshawytcha) escapement in lower Columbia River (LCR) tributaries since the 1940s. These early spawning ground surveys typically consisted of one to three annual surveys conducted within an index reach where lives, deads, and redds were enumerated while sampling carcasses for biological data. Escapement estimates were generated by taking the peak count (the largest single day combined count of lives and deads) within the index area and multiplying by an expansion factor (herein after referred to as a peak count expansion factor (PCEF)) specific to each basin. The peak count expansion factor was either based on previous mark-recapture study done within the basin or professional judgement. These methods allowed for escapement estimates to be generated for several basins with minimal effort. In 1999, Chinook salmon in the LCR Evolutionarily Significant Unit (ESU) were listed as threatened under the Endangered Species Act. As a result, WDFW began to strategically implement more intensive monitoring for adult fall Chinook salmon on select populations over the next decade in an effort to improve estimates. In 2010, WDFW further modified and expanded its existing fall Chinook salmon escapement monitoring program. This was to ensure estimates of viable salmonid population parameters were being generated for all populations within the LCR ESU and that these parameters met the National Oceanic and Atmospheric Administration Fisheries guidelines for accuracy and precision. As a result of these new, robust fall Chinook salmon escapement estimates, we were able to generate new PCEFs, often based on data from multiple years, and update historical escapement estimates with the associated uncertainty. We developed 15 new basin and stocks-pecific PCEFs and updated 746 fall Chinook salmon escapement estimates between 1943 and 2009. When possible, we used mark-recapture, Area-Under-the-Curve based on counts of live fish, or redd-based estimates to develop estimates of escapement. However, most (92%) of the estimates were based on peak count expansion methods due to a lack of data available.