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Trap-and-haul is a mitigation strategy at many hydropower dams lacking upstream fish-passage facilities, and protocols are needed to maximise its effectiveness. We used biotelemetry to assess the potential benefits of releasing transported adult Chinook salmon (Oncorhynchus tshawytscha) into a cold-water reservoir v. a relatively warm-water tributary before spawning. Over 5 years, we released 160 salmon into Foster Reservoir (Oregon, USA) and another 102 into the South Santiam River near historical salmon spawning areas further upstream. In total, 70% of reservoir-released salmon entered an upriver tributary after spending a median of 3–95 days annually in the reservoir. Data recovered from 61 archival temperature loggers indicated that salmon were ~3–6°C cooler per day in the reservoir than in the river. We estimated that cumulative exposure of reservoir-released fish was reduced by 64 degree days, on average (range=–129 to 392), relative to river-released fish. Release into the reservoir was not risk free; 14% of all reservoir-released fish fell back downstream v. 1% of river-released fish. We conclude that reduced transport distance, reduced thermal exposure and potential survival benefits of releasing salmon into reservoirs should be weighed against risks of factors such as fallback and homing errors.

Episodically high adult mortality during migration and near spawning sites has hindered the recovery of threatened spring‐run Chinook Salmon Oncorhynchus tshawytscha in Oregon's Willamette River basin. In 2011–2014, we assessed migration mortality for 762 radio‐tagged adults along a ~260‐km reach of the main stem of the Willamette River. Annual survival of salmon to spawning tributaries ranged from 0.791 (95% CI = 0.741–0.833) to 0.896 (0.856–0.926), confirming concerns about mortality in the migration corridor. In a series of general linear models, descaling, marine mammal injuries, and head injuries to adult Chinook Salmon were linked to reduced survival during migration to tributaries. Many injuries were minor (i.e., epidermal abrasions), which we hypothesize were unlikely to have caused direct mortality but may have increased salmon vulnerability to pathogens or other disease processes. Mortality in the main stem was not significantly associated with salmon body size, energetic status, sex, origin (hatchery, wild), river discharge, or water temperature metrics. The ~10–21% estimates of en route mortality in this study provide an important benchmark for the main stem of the Willamette River. The estimates complement ongoing efforts to quantify mortality of adult Chinook Salmon in Willamette River tributaries and after collection and transport to spawning sites above high‐head hydroelectric dams. Received September 15, 2016; accepted December 1, 2016 Published online March 3, 2017

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