Advances in Fisheries Bioengineering

<em>Abstract.</em>—Although many hydroelectric dams have fishways for upstream passage of migratory fish, passage delays often occur at these sites. Migrational delay may affect fish detrimentally in several ways, including depletion of energy reserves, suboptimal arrival timing at spawning sites, and prolonged exposure to hazardous conditions at the face of dams. We applied time-to-event analyses to passage times of radio-tagged adult Chinook salmon <em>Oncorhynchus tshawytscha </em>at four dams on the lower Columbia River, where many fish require several days to pass each dam. The analysis allowed us to determine instantaneous passage rates in response to fluctuating river conditions. By relating variability in passage rate to the predictor variables river temperature, river flow, and fish size, we determined the relative contribution of various factors to the passage time of migrating fish. We fit the model by maximizing the likelihood function that incorporated information from individuals rather than aggregated groups of fish. We used Akaike’s Information Criterion to distinguish among several competing models, each of which used a different predictor variable. We found that daytime passage rates were significantly greater than nighttime passage rates. Also, the influence of river flow, river temperature, and fish length on passage rates varied at the four dams. However, when a factor had a significant influence on passage time, the direction of the relationship was consistent across dams: river flow and fish length were positively related to passage time (greater values led to longer passage time), and river temperature was negatively related. This method is easily adaptable to study passage time of any fish population facing a broad range of obstacles to migration, whether natural or man-made.