Prespawn Mortality of Spring Chinook Salmon in Three Willamette River Populations

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Abstract
Impassable dams on major tributaries to the Willamette River, Oregon, have restricted access to historical spawning habitat for returning adult Chinook Salmon Oncorhynchus tshawytscha. To restore these populations, some fish are collected and transported in trucks upstream of dams (i.e., adult trap-and-haul operations). However, persistently high prespawn mortality (PSM) rates in these programs have hindered conservation efforts. The objective of this study was to evaluate environmental factors and individual fish traits associated with PSM in Chinook Salmon populations that were transported and released upstream of dams in three Willamette River tributaries. Annual PSM of tagged and untagged female salmon ranged from 6% to 88% across 7 years. Mortality was higher in warmer years and in warmer tributaries, consistent with rangewide studies of PSM in Chinook Salmon. Effects of individual traits were mixed across sites: longer salmon and those released relatively early or late in the migrations had higher mortality. We conclude that strategies designed to minimize adult Chinook Salmon exposure to warm water temperatures, as well as additional research into both proximate and ultimate causes of PSM, may lead to better management and conservation outcomes for outplanted populations, including those in Willamette River tributaries.

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