Migratory Behavior of Adult Spring Chinook Salmon in the Willamette River and its Tributaries: Completion report

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Abstract
Migration patterns of adult spring chinook salmon above Willamette Falls differed depending on when the fish passed the Falls, with considerable among-fish variability. Early-run fish often terminated their migration for extended periods of time, in association with increased flows and decreased temperatures. Mid-run fish tended to migrate steadily upstream at a rate of 30-40 km/day. Late-run fish frequently ceased migrating or fell back downstream after migrating 10-200 km up the Willamette River or its tributaries; this appeared to be associated with warming water during summer and resulted in considerable mortality. Up to 40% of the adult salmon entering the Willamette River System above Willamette Falls (i.e. counted at the ladder) may die before reaching upriver spawning areas. Up to 10% of the fish passing up over Willamette Falls may fall-back below the Falls; some migrate to the Columbia River or lower Willamette River tributaries. If rearing conditions at hatcheries affect timing of adult returns because of different juvenile development rates and improper timing of smolt releases, then differential mortality in the freshwater segment of the adult migrations may confound interpretation of studies evaluating rearing practices.

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