Comparison of selected population characteristics of adult Chinook salmon during upstream passage through a resistance board weir and during carcass surveys

We compared population characteristics of adult Chinook salmon (Oncorhynchus tshawytscha) during 2003-2006 in the lower Stanislaus River, Stanislaus County, California, by counting and measuring live fish moving past a resistance board weir and dead fish counted and measured by the California Department of Fish and Wildlife (CDFW) during annual carcass surveys. The comparison of annual escapement was extended to 2007-2009 by including unpublished data. Although annual salmon passage counts at the weir were significantly correlated with estimates of carcass survey escapement, size estimates of live fish passing the weir were smaller on average than dead fish measured during carcass surveys. Sex ratios also differed for fish counted at the weir compared to those counted during carcass surveys. In general, females outnumbered males in both datasets, except in 2004 when more males than females were counted at the weir. Ratios of clipped to unclipped adipose fins differed significantly between fish from the weir and from the carcass surveys during 2005-2006, but not during 2004. These results suggest that population characteristics of adult salmon returning to the Stanislaus River may be better represented by the relatively high numbers of live fish examined during their concentrated passage through the weir than by the lower numbers of widely dispersed dead fish examined during carcass surveys.