Quantifying the Uncertainty of a Juvenile Chinook Salmon Race Identification Method for a Mixed-Race Stock

Expected daily FL ranges (length at date) of juvenile Chinook Salmon Oncorhynchus tshawytscha have been used throughout California's Central Valley to identify federally listed winter-run and spring-run juveniles in a mixed four-race stock. Accurate race identification is critical both to species recovery and to management of the water supply for 25 million people and a multibillion-dollar agricultural industry. We used genetic race assignment of 11,609 juveniles sampled over 6 years to characterize the accuracy of the length-at-date approach, specifically by testing two of its central assumptions: (1) juvenile FL distributions do not overlap between races on a daily basis; and (2) the growth rates that are used to project FL at date are accurate. We found that 49% of FLs for genetically identified juveniles occurred outside the expected length-at-date ranges for their respective races, and we observed a high degree of overlap in FL ranges among the four races. In addition, empirical growth rates were well below those from which length-at-date criteria were derived. Given the high degree of FL overlap between races, we conclude that modification of the length-at-date method will not substantially reduce identification error. Thus, we recommend that genetic assignment be used at least as a supplemental approach to improve Central Valley Chinook Salmon race identification, research, and management.

Received January 7, 2014; accepted July 22, 2014