Impacts of Dam and Reservoir Passage on Outmigrating Juvenile Chinook Salmon: Results from a Paired Release Study in the Upper Willamette Basin

Upper Willamette spring Chinook salmon, Oncorhynchus tshawytscha, are listed as threatened under the federal Endangered Species Act.  Poor survival of juveniles through high-head dams and associated reservoirs has been cited as a primary threat to their recovery, but dam and reservoir impacts are not well quantified.  To help evaluate broad-scale impacts among years and subbasins, we released PIT-tagged juvenile hatchery spring Chinook salmon (median 68 mm fork length) above and below dams in the Middle Fork Willamette (MFW) and North Santiam (NS) rivers from 2011-2014.  Detection proportions downstream of the release sites at Willamette Falls were greater for the below-dam releases across years and basins and were significantly different (P<0.05) from expected proportions.  Detection proportion effect sizes for below-dam versus above-dam release comparisons ranged from 60-94% in the MFW and from 15-39% in the NS.  Movement rates for fish released below dams were often significantly faster than for above-dam releases, and Chinook released above dams grew significantly faster than those released below dams.  We concluded juvenile Chinook salmon produced above the dams experience substantial mortality, and hypothesized that dam passage and predation in the reservoirs are the most likely sources, though accelerated reservoir growth may provide survival benefits.