Canadian‐Origin Chinook Salmon Rearing in Nonnatal U.S. Tributary Streams of the Yukon River, Alaska

Abstract

AbstractYukon River Chinook salmon Oncorhynchus tshawytscha are described as having a “stream‐type” life history strategy. After emergence from river gravel, juveniles typically feed and grow in tributary streams of the Yukon River throughout their first summer, overwinter in freshwater, and usually leave their rearing areas for marine waters during the second spring or summer. Nonnatal rearing has been described in the upper Canadian portion of the drainage, but information is lacking for downstream U.S. waters. In 2006–2007, a study was conducted to document nonnatal rearing and the genetic origin of Chinook salmon in U.S. tributary streams of the Yukon River. Eight nonspawning streams were selected for study, seven located in a 260‐km segment between the U.S.–Canada border and Circle, Alaska, and one located 742 km downstream from the border. Age‐0 juveniles were captured in all eight streams. Genetic stock composition analyses using 13 standardized microsatellite loci assigned the fish to Canadian source populations. The Carmacks region (over 460 km upstream from the border) contributed 91% to the mixtures in 2006 and 82% in 2007. Canadian stocks nearest the border and from large river systems were underrepresented in the collections. Simulation and known‐origin mixture analyses demonstrated that stock composition and individual assignment estimates derived from the genetic baseline were accurate and precise. Some juveniles may have traveled over 1,200 km to reach rearing areas in U.S. waters. Future studies would help define the importance of this life history strategy to the overall health and productivity of Yukon River Chinook salmon.