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Abstract.-The relationships between lower Columbia River water temperatures and migration rates, temporary tributary use, and run timing of adult fall Chinook salmon Oncorhynchus tshawytscha were studied using historical counts at dams and recently collected radiotelemetry data. The results from more than 2,100 upriver bright fall Chinook salmon radio-tagged over 6 years (1998,(2000)(2001)(2002)(2003)(2004) showed that mean and median migration rates through the lower Columbia River slowed significantly when water temperatures were above about 208C. Slowed migration was strongly associated with temporary use of tributaries, which averaged 2-78C cooler than the main stem. The proportion of radio-tagged salmon using tributaries increased exponentially as Columbia River temperatures rose within the year, and use was highest in the warmest years. The historical passage data showed significant shifts in fall Chinook salmon run timing distributions concomitant with Columbia River warming and consistent with increasing use of thermal refugia. Collectively, these observations suggest that Columbia River fall Chinook salmon predictably alter their migration behaviors in response to elevated temperatures. Coolwater tributaries appear to represent critical habitat areas in warm years, and we recommend that both main-stem thermal characteristics and areas of refuge be considered when establishing regulations to protect summer and fall migrants.

The relationships among behavior, environment, and migration success in anadromous fishes are poorly understood. We monitored migration behavior at eight Columbia and Snake river dams for 18 286 adult Chinook salmon (Oncorhynchus tshawytscha) and steelhead (sea-run Oncorhynchus mykiss) over 7 years using radiotelemetry. When statistically controlling for variation in flow, temperature, fisheries take, and other environmental variables, we observed that unsuccessful individuals — those not observed to reach spawning areas — had longer passage times at nearly all dams than fish that eventually reached tributaries. In many cases, times were also longer for unsuccessful adults passing through a multiple-dam reach. Four ecological mechanisms may have contributed to these patterns: (i) environmental factors not accounted for in the analyses; (ii) inefficient responses by some fish to passage conditions at dams that resulted in slowed passage, energetic depletion, and unsuccessful migration; (iii) ongoing selection for traits needed to pass obstructions; and (or) (iv) passage rate was not directly linked to migration success, but rather, both resulted from relatively poor phenotypic condition upon river entry in unsuccessful migrants. Overall, these results illustrate the need for a mechanistic understanding of the factors that influence migration success and the need for fitness-based criteria to assess the effects of dams on anadromous fishes.

We assessed upstream migration rates of more than 12,000 radio-tagged adult Chinook salmon Oncorhynchus tshawytscha and steelhead O. mykiss past a series of dams and reservoirs on the Columbia and Snake rivers. Most fish passed each dam in less than 2 d. Migration behavior in reservoirs and through multiple dam-reservoir reaches varied within and between years and between species. Within years, spring-summer Chinook salmon migrated more rapidly as water temperature and date of migration increased; between years, spring-summer Chinook salmon migrated fastest in low-discharge years. Steelhead migrations slowed dramatically when summer water temperatures peaked within each year, then increased as rivers cooled in fall. Mean summer temperatures explained more between-year variation in steelhead passage rates than did differences in discharge. Fall Chinook salmon migration rates also slowed during periods of warm water. Protracted passage times within the hydrosystem were most likely for fish from all runs that fell back over and reascended dams and for steelhead that sought thermal refugia by straying temporarily into coldwater tributaries.