Low Yukon River Chinook salmon returns in recent years have resulted in commercial fishery closures and severe restrictions to subsistence fishing in an effort to meet escapement goals. Causes of reduced productivity and poor returns are unknown, and have led to several initiatives to improve assessment and understanding of the mechanisms driving these declines. Thiamine (vitamin B1) deficiency has led to population-level productivity changes and dramatic declines in salmonid stocks from the Great Lakes and Baltic Sea, and is a possible mechanism influencing productivity of Yukon Chinook salmon. Thiamine deficiency increases early life stage mortality of offspring, lowers adult survival, and reduces vigor and migratory abilities, particularly for stocks with long migration routes like Canadian-origin Chinook salmon. A preliminary analysis completed in 2012 was limited in samples, but indicated potential for thiamine deficiency in Yukon Chinook salmon (only 23% of the females sampled in the Yukon River had eggs with fully replete thiamine levels). The proposed work will increase sample sizes and provide an interannual context for variability in prevalence of thiamine deficiency. Our goal is to assess Chinook salmon egg thiamine levels and to assess the potential for thiamine deficiency to influence Canadian-origin Yukon Chinook salmon productivity. We are requesting funds to analyze Chinook salmon eggs sampled from predominantly mixed Canadian-origin stocks (Rampart Rapids) and from two Canadian spawning locations (Teslin River and Whitehorse Rapids Hatchery). We hypothesize that thiamine levels in Canadian-origin Yukon Chinook Salmon are below thiamine replete levels in some or all areas.  At the end of this project, we will be able to specify to the Yukon River Panel what areas, if any, Canadian-bound Yukon Chinook salmon are thiamine deficient. This research could increase our understanding of Chinook salmon productivity declines in the Yukon River.