RESEARCH ARTICLE Migratory Patterns of Wild Chinook Salmon Oncorhynchus tshawytscha Returning to a Large, Free-Flowing River Basin

Upriver movements were determined for Chinook salmonOncorhynchus tshawytscha re-turning to the Yukon River, a large, virtually pristine river basin. These returns have declined dramatically since the late 1990s, and information is needed to better manage the run and facilitate conservation efforts. A total of 2,860 fish were radio tagged during 2002–2004. Most (97.5%) of the fish tracked upriver to spawning areas displayed continual upriver movements and strong fidelity to the terminal tributaries entered. Movement rates were sub-stantially slower for fish spawning in lower river tributaries (28–40 km d-1) compared to upper basin stocks (52–62 km d-1). Three distinct migratory patterns were observed, includ-ing a gradual decline, pronounced decline, and substantial increase in movement rate as the fish moved upriver. Stocks destined for the same region exhibited similar migratory pat-terns. Individual fish within a stock showed substantial variation, but tended to reflect the re-gional pattern. Differences between consistently faster and slower fish explained 74 % of the within-stock variation, whereas relative shifts in sequential movement rates between “hares ” (faster fish becoming slower) and “tortoises ” (slow but steady fish) explained 22 % o