Influence of Fishway Placement on Fallback of Adult Salmon at the Bonneville Dam on the Columbia River

Abstract

Using radiotelemetry, we observed and quantified the behavior of upstream migrating adult Chinook salmon Oncorhynchus tshawytscha and sockeye salmon O. nerka exiting the Bradford Island fishway at the Bonneville Dam on the Columbia River in 1997 and 1998. Nearly all of the fish that exited the fishway migrated upstream along the Bradford Island shoreline. Those fish that took the route nearest to the spillway were most likely to fall back over the spillway. From 14.5% to 21.3% of the fish tracked along the Bradford Island shore fell back over the spillway of the dam. The combined effects of spill, water temperature, and Secchi disk visibility were associated with route patterns and fallback behavior during each year. High spill was significantly and positively correlated with fallback behavior for Chinook salmon in 1998. Most of the fish we tracked that fell back reascended the fishway and migrated upstream (≥95% in 1997; ≥70% in 1998). We suggest that modifying the configuration of this fishway's exit would decrease the proportion of fish that fall back, perhaps reduce the risk of injury and fatigue, and improve the precision of counts of fish migrating upstream.