Physiological and behavioral changes occur in the spring when juvenile Pacific salmon Oncorhynchus spp. undergo smolting. Survival is maximized if the timing of these changes coincides with migration from fresh to marine environments. Therefore, understanding how environmental conditions influence the onset, duration, and termination of smolting can have substantial management implications, particularly for flow-controlled rivers. We used an information-theoretic model comparison analysis to investigate the roles of daily mean temperature, temperature experience (accumulated thermal units [ATU]), photoperiod, and flow on the timing of the downstream migration of Chinook salmon O. tshawytscha smolts from the Nechako River in central British Columbia. Both binary (migration or not) and count (the total number of migrants) models were developed that predicted the downstream migration of Chinook salmon based on data collected from fish captured at rotary-screw traps from 1992 to 2004. The analyses identified a combination of temperature experience, flow, and the number of spawners as best able to describe the observed migration patterns. In addition, increasing ATU had a positive influence on migration, while increasing flow had a negative influence. Temperature experience was found to have more influence on migration than daily mean temperature. The predictive ability of each model was tested with 2 years of independent data. The count model accurately predicted the general trends in migration and, in particular, the termination of migration, but not the daily fluctuations in movement. By contrast, the binary model predicted whether fish would migrate on a given day with accuracies of 93% and 99%, respectively, for the 2 years tested. Temperature experience was more strongly linked to migration than the daily or threshold temperature; warmer temperatures resulted in earlier migration. Our data suggest that flow plays an important role once migration is under way and may even serve as a termination cue. Furthermore, the number of migrants and the probability of migration was positively related to the number of spawners. Based on the results of this study, flow manipulations that change the timing, duration, or magnitude of temperature and flow in the spring could affect the migration of Chinook salmon. Both temperature and river flow should be considered when one is managing flow-controlled watersheds for salmon productivity.

We investigated the role of temperature and current on migration pattern and physiological change in Chinook salmon ( Oncorhynchus tshawytscha ) smolts. Chinook parr were implanted with passive integrated transponder tags and placed in circular tanks with directional current. Paired antennas in each tank allowed for the monitoring of movements with or against the current. Two temperature regimes, naturally increasing (warm) and constant (cool), were combined with high current (>0.5 m·s–1) and low current (<0.1 m·s–1) to create four experimental treatments. At 2-week intervals, fish were sampled for gill Na+,K+-ATPase activity and plasma cortisol concentration. Increasing temperature resulted in an earlier peak in movement than colder groups and showed a distinct peak in gill Na+,K+-ATPase activity. Current did not influence physiological parameters associated with smolting and was not found to initiate movement; movement was related to accumulated thermal units (ATU). The presence of a strong, directional current, however, resulted in a period of more defined movement, suggesting a possible influence of current once migration is underway. A combination of either photoperiod or ATU with gill Na+,K+-ATPase was most strongly linked to movement. ATU was also found to be more strongly correlated with the smolting process than daily mean temperature.

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