Does Passage through Snake River Dams Cause Latent Mortality?

From 2005 to 2011, NOAA Fisheries collected outmigrating hatchery stream-type Chinook salmon at Lower Granite Dam and implanted Passive Integrated Transponder (PIT) tags to test whether passage through the lower three Snake River dams caused mortality that was not expressed until after fish had completed migration through the hydrosystem (“latent mortality”).

The study design included three treatment groups: one trucked from Lower Granite Dam and released below Ice Harbor Dam (reference group); one transported by truck for an equivalent amount of time and returned to Lower Granite Dam for release into the tailrace (dam-passage group); and one released into the tailrace of Lower Granite Dam without having been trucked (truck-effects group).  We attempted to tag 10 replicates each year from mid-April to mid-May, with two days of tagging per replicate.  Comparison of groups was based on smolt-to-adult return rates, using estimated numbers of smolts from each group surviving to the tailrace of McNary Dam, the first dam in common among all three groups after release, and numbers of returning adults detected at Bonneville Dam, the first dam encountered by returning fish migrating up the Columbia River.