Effects of Transportation and Other Factors on Survival Estimates of Juvenile Salmonids in the Unimpounded Lower Columbia River

We estimated the survival of juvenile salmonids out‐migrating through the lower Columbia River to the Pacific Ocean. We tested the null hypotheses that no association exists between survival and transportation type (including barge transportation and in‐river migration with no transportation), release date, river flow, mean body weight, and tag type. During 2002‐2004, spring‐summer (SS) and fall Chinook salmon Oncorhynchus tshawytscha and steelhead O. mykiss were implanted with uniquely coded radio or acoustic transmitter tags and released during the early, middle, and late out‐migration periods. A series of receiver lines were used to detect these fish and estimate survival. Estimated survival varied considerably with transportation type and release date for fall Chinook salmon and steelhead, but the results were dependent upon year. Estimated survival between the lowermost dam (Bonneville Dam) and the upper estuary (river kilometer 46) was relatively high for SS Chinook salmon and steelhead, whereas estimates for fall Chinook salmon were comparatively low. This order of high to low survival for SS Chinook salmon, steelhead, and fall Chinook salmon corresponds with the chronology of out‐migration timing and waning river flows. After accounting for a number of factors, estimated survival between Bonneville Dam and the upper‐estuary site did not differ significantly between steelhead that were barged and those that migrated in‐river during 2002 and 2003. Conversely, estimated survival to the upper‐estuary site was quite low for barged fall Chinook salmon in comparison with fish migrating in‐river during the middle and late release dates in 2003. Hypotheses for the low survival of barged fall Chinook salmon are presented and discussed.