Population Composition, Migration Timing, and Harvest of Columbia River Chinook Salmon in Late Summer and Fall

Abstract

We used radiotelemetry to evaluate population composition, run timing, and reservoir harvest patterns for adult Columbia River fall-run Chinook salmon Oncorhynchus tshawytscha. Chinook salmon (n = 5,886) were collected at Bonneville Dam during August–October over 7 years. We selected for upriver bright (URB) populations en route to interior basin spawning sites because these groups are priority populations for both fisheries and conservation efforts. Run composition varied within and among years, but in all years a relatively large percentage of the earliest migrants returned to upper Columbia River sites and the majority of late-run fish returned to the Columbia River Hanford Reach. Deschutes, Yakima, and Snake River populations typically constituted small (≤17%) but relatively constant proportions of the run throughout each migration season. Population-specific migration timing distributions indicated modest but persistent timing differences among populations, particularly for Hanford Reach and upper Columbia River populations. Annual reported reservoir harvest estimates ranged from 12% to 26%. Harvest rates varied seasonally within years, from relatively low mean rates (≤11%) for fish tagged early and late in migrations to peak rates of over 25% for those tagged in late August and early September. These patterns suggest that it may be possible to increase harvest of abundant populations and reduce harvest of some vulnerable populations by adjusting the timing of fisheries. In addition, there was evidence that larger fish were harvested at higher rates and that mean fish size differed among populations. The combined results improve our understanding of the Columbia River URB fall Chinook salmon run and should help in refining harvest and escapement management plans.