PARENT SESSION
Contributed Oral Session 38: Aquatic Faunal Assemblage Responses to Stressors
Tuesday, August 9, 8:00 AM - 11:30 AM, Meeting Room 513 C, Level 5, Palais des congrès de Montréal

Unsuccessful migration in Columbia River salmonids associated with slow dam passage: Indirect negative effects of dams or condition dependent mortality?

Caudill, Christopher ^*,1, Daigle, William ¹, Keefer, Matt¹, Jepson, Mike¹, Burke, Brian², Peery, Christopher ¹, ¹ Department of Fish and Wildlife, Moscow, Idaho, USA² NOAA-Fisheries, Seattle, Washington

ABSTRACT- The construction of large dams has been implicated along with other factors in the decline of Columbia Basin salmon and steelhead. We used event-time analysis to assess how passage times of adult salmonids at up to eight dams in the lower Columbia and Snake Rivers were related to physical passage conditions and fish fate. The models provide several advantages over linear models, and passage time is expressed as a probability or odds ratio rather than as a continuous variable. Results from 11,593 radio-tagged spring, summer, and fall-run Chinook salmon and steelhead over 7 years revealed patterns which were largely similar among runs. Time of day was the strongest predictor of passage probability, because few fish pass at night. Fish had slightly longer passage times during conditions of higher discharge over the dam spillway. Responses to temperature were more complex, and relationships were generally weak. The final fate of fish was significantly related to passage probability at nearly all projects and all runs, where fish with unknown fates had lower passage probabilities (and longer passage times) at individual projects than fish that reached spawning tributaries. Review of the detailed telemetry records for nearly 3,000 fish with unknown fates suggest the majority of these fish expired without passing the dam they last encountered. Overall, these patterns suggest three potential mechanisms that could create the observed relationship between slow passage and unknown fate: that behavioral variation among fish leads to long passage time, depletion of energetic stores, and poor migration performance; that fish with long passage times were in poor condition prior to river entry and poor migration performance was independent of relatively slow dam passage; or that these two mechanisms have combined negative effects on salmonid migration performance.

Key words: salmon, time-event analysis, regulated rivers, indirect effects