Comparison of DIDSON Sonar Based Estimates of Chinook Salmon Escapement with Other Methods in the Coweeman River, Washington

The Coweeman River supports a small spawning population of Tule fall Chinook salmon (200-2000 adults), which are listed for protection under the US Endangered Species Act but harvested in mixed stock Pacific Ocean and mainstem Columbia River fisheries.  The Coweeman Chinook stock has a low proportion of hatchery spawners and has been used as an index population for other less intensively monitored Tule populations.  Over the last decade the Washington Department of Fish and Wildlife (WDFW) and partners have funded numerous studies to determine effective methods to accurately estimate Chinook salmon escapement for this population.  From 2007 to 2009, a DIDSON sonar was operated below the major spawning area to estimate escapement.  Concurrently, independent escapement estimates were made using more traditional methods such as mark-recapture, redd counts, and Area-Under-the Curve (AUC) surveys.  Bayesian approaches to estimate salmon escapement for all methods will be presented, with an emphasis on the use of mixture models for sonar-based estimates and a comparison of the sonar estimates with more traditional approaches.