Holding and Pre-Spawning Behavior of Adult Spring Chinook Salmon in the Upper Yakima River; Insights from Radio-Telemetry

We used radio telemetry to document movements of adult spring Chinook salmon (Oncorhynchus tshawytscha) in the upper Yakima River during their spawning migration.  Adult salmon (total length 60-93 cm) were captured as they migrated into the upper Yakima River at an advanced stage of their spawning migration and implanted intragastrically with radio transmitters.  Radio-tagged fish were relocated between June and October, 2008 using a combination of fixed site receivers and mobile tracking surveys. A high proportion of radio-tagged fish either did not move upstream, or fell back from the release site, up to 17 km.  It is suspected that stress associated with handling during the late stages of migration combined with high temperatures contributed to mortality and migrational behavior alteration. Those fish that migrated from the release site to points upstream (mean distance 85 km; range, 46-110 km) moved at a mean rate of 3 km/day (range 0.5-6.2).  Most of these fish were tracked to large, deep pools where they held from late June to late August.  Many fish subsequently made final movements upstream (as much as 25 km) to spawning areas in early September, a period that coincided with managed flow reductions and final maturation.  Preliminary results suggest that spring Chinook in the upper Yakima River may thermoregulate behaviorally and are especially sensitive to environmental and manmade stressors during the late stages of their spawning migration.