Estimates of Fall-run Chinook Salmon Escapement in Two San Joaquin River Tributaries from Device-based and Survey-based Methods

Monitoring the escapement (i.e., spawner abundance) of adult salmonids is a fundamental component of fisheries stock assessment and evaluating recovery goals. Whereas multiple methods exist for estimating escapement, they exhibit trade-offs between degree of accuracy and implementation expenses and effort. Various methods are used throughout the Central Valley to estimate escapement of the four runs of Chinook Salmon (Oncorhynchus tshawytscha), but few rivers have more than one escapement monitoring program. Fall-run Chinook Salmon escapement to the Stanislaus and Tuolumne rivers is estimated by two independent monitoring programs on each river. Since 1952, California Department of Fish and Wildlife has performed carcass surveys that provide an index of escapement. Beginning in 2003 (Stanislaus River) and 2009 (Tuolumne River), private contractors have seasonally installed and operated weirs with fish counting devices to enumerate returning adults. We used the overlapping time series to evaluate the relationship between estimates from carcass surveys and the counting weirs. With few exceptions, estimates from counting adult salmon at the weirs as they entered the spawning grounds were greater than estimates from carcass surveys on both rivers. Escapement estimates from the Stanislaus weir were on average 1.7 (SE = 0.07) times greater than estimates from carcass surveys, while estimates at the Tuolumne weir were 2.5 (SE = 0.17) times greater than estimates from carcass surveys. Estimates from both methods were highly correlated, with r2 values > 96% for both rivers. The high degree of covariation between methods indicates escapement estimates are robust and can be used in population models that incorporate additional data sources, such as estimates of juvenile production. Having multiple sources of data on fall-run Chinook Salmon will be increasingly useful for guiding and evaluating management actions in these heavily managed rivers.