Fallback, reascension, and adjusted fishway escapement estimates for adult Chinook salmon and steelhead at Columbia and Snake River dams

During their upstream spawning migration in the Columbia River basin, some adult salmonids (Oncorhynchus spp.) ascend and then fall back over main-stem hydroelectric dams. Fallback can result in fish injury or death, migration delays, and biases in fishway counts, the primary index for escapement and the basis for production estimates and harvest quotas. We used radiotelemetry to calculate fallback percentages and rates, reascension percentages, biases in fishway escapement estimates due to fallback, and occurrence of behaviourally motivated fallback (correcting overshoot of natal sites) by spring–summer and fall Chinook salmon (O.tshawytscha) and steelhead (O. mykiss). The study area included eight Columbia River and Snake River dams evaluated from 1996 to 2001. For all years combined, about 22% of spring-summer Chinook salmon, 15% of fall Chinook salmon, and 21% of steelhead fell back at least once at a dam. Fallback percentages for spring–summer Chinook salmon were generally highest at Bonneville and the Dalles dams and decreased at progressively upstream dams. Fallback rates for spring–summer Chinook salmon were positively correlated with river discharge. Fallback percentages for steelhead and fall Chinook salmon were less variable between years but were more variable between dams than those of spring–summer Chinook salmon. Reascension percentages at dams ranged widely between runs and sites and were negatively related to the number of fish that entered tributaries downstream from the fallback location. Fall Chinook salmon were the most likely to enter a downstream tributary after falling back, though this behaviour was also observed in spring–summer Chinook salmon and steelhead. For all years and at all dams, fallback produced positive fishway count biases ranging from 1% to 16% for spring-summer Chinook salmon, 1% to 38% for fall Chinook salmon, and 1% to 12% for steelhead