A Long, Long Way from Home - Canadian-Origin Chinook Salmon Rearing in Nonnatal U.S. Tributary Streams of the Yukon River, Alaska

The Yukon River drainage encompasses over 850,000 km² and provides important subsistence and commercial salmon fisheries along most of its 3,200 km length.  Chinook salmon travel the furthest upstream of the five species of salmon present in the Yukon River, spawning up to 2,900 km from the mouth.  Approximately 50% of Chinook salmon entering the Yukon River each year are bound for distant spawning grounds in Canada.  Yukon River Chinook salmon are stream-type, typically leaving for marine waters during their second year.  Canadian-origin juveniles use both natal and nonnatal Canadian streams for rearing, but in downstream U.S. waters, only natal rearing had been documented.  This presentation describes results from recent juvenile distribution studies (USFWS) in the upper U.S. portion of the drainage that added significantly to our understanding of juvenile migratory behavior.  In 2006 and 2007, eight representative non-spawning tributary streams were sampled for rearing Chinook salmon and genetic origins of captured fish were determined using mixed-stock and individual assignment analyses.  This pilot study revealed that all sampled streams contained Canadian-origin juveniles, with the vast majority (91% in 2006 and 82% in 2007) coming from one of seven Canadian stock groups (see TAFS 2011).  In 2008, a comprehensive 3-year study was initiated to inventory non-spawning streams for the presence of juvenile Chinook salmon between the U.S./Canada border and the Tanana River confluence (850 km below the border).  Fifty-six streams were sampled, 44 streams contained juveniles, and 616 genetic samples were collected.  Genetic results were similar to the 2006-2007 study; with the addition of the upper U.S. stock group in some downstream samples.  Some juveniles traveled over 1,300 km from their natal streams, representing the longest recorded dispersal distances for subyearling stream-type Chinook salmon in North America.  Catch data coupled with weather observations suggested that some fish switched rearing streams during the summer, likely from high stream flows (rain events) displacing juveniles downstream.  Results from two tributary trapping stations, located 800 km apart, suggested a prolonged downstream dispersal of some individuals.  As these studies demonstrate, migratory behavior of juvenile salmonids can be complex.  Outmigrant juvenile monitoring projects will benefit, in both study design and data interpretation, from a more complete understanding of stream-specific migratory habits of the targeted species.