Temperature-sensitive transmitters and archival tags allowed precise measurement of adult Chinook salmon Oncorhynchus tshawytscha responses to high water temperatures and other environmental variables in the Klamath River basin during 2004 and 2005. Mean daily river temperatures upon initiation of upriver migration by adult Chinook salmon after a period of thermally induced migration inhibition ranged from 21.88C to 24.08C (mean ¼ 22.98C). During the first week (168 h) of migration, mean average body temperature was 21.98C, mean average minimum daily body temperature was 20.68C, and mean average maximum daily body temperature was 23.18C. Temperatures above these levels appeared to completely block migration in almost all circumstances. Migration was inhibited at lower mean daily river temperatures during periods of increasing river temperatures than during periods of declining river temperatures. The ability of adult Chinook salmon to correctly gauge the onset of periods of declining or inclining river temperature is a vital mechanism for taking advantage of brief thermal windows for upriver migration. Weather fronts were responsible for periods of declining river temperature, but no precipitation or consistent drops in atmospheric pressure were associated with these weather fronts or with fish movements. The associated decrease in light levels, however, appeared to serve as an indicator of impending reductions in river temperatures for adult Chinook salmon. Changing river discharge had a negligible influence on migration behavior, and hypoxia sufficient to inhibit upriver migration (i.e., dissolved oxygen , 5 mg/L) never occurred during the study. The upper thermal limits to adult Chinook salmon migration as indicated by results from the Klamath River basin are substantially higher than previously reported in the literature and approached or exceeded the highest ultimate upper incipient lethal values determined for any life stage of this species.

Biotelemetry and body temperature records of 130 adult Chinook salmon Oncorhynchus tshawytscha tagged over multiple years were used to investigate migration behaviors in response to diverse riverine conditions within the context of run timing strategies in a single river system. Four distinct runs were identified: Klamath–Trinity spring run, Trinity summer run, Klamath fall run, and Trinity fall run. Only the timing of Klamath–Trinity spring‐run and Trinity fall‐run Chinook salmon tended to prevent fish exposure to adversely high river temperatures. Faster migration rates reduced the accumulation of thermal units regardless of river temperature, especially at migration rates ≥10 km/d. Behavioral thermoregulation was minimal for all run groups (8% of all migrants), typically brief (<12 h) during summer months at nonnatal tributary confluences. Spring‐run fish migrating on the descending limb of the snowmelt peak experienced increased accumulated thermal units due to slow migration rates, resulting in more elevated relative disease risk with lower apparent migration success than rapidly migrating summer‐run fish. Klamath fall‐run fish held extensively in the lower Klamath River regardless of date or river condition, resulting in the highest cumulative thermal experience of all run groups with moderate thermal stress. Trinity fall‐run fish experienced minimal thermal stress due to their later run timing. Results demonstrate the need to comprehensively investigate fish migrations in order to detect potentially subtle but important context‐sensitive dynamics between migration behaviors and environmental conditions that can impact the viability of salmon populations. This study's findings highlight concerns regarding global warming and vulnerability runs of salmon that migrate in the late spring and early fall in the absence of adaptive shifts in run timing or appropriate human intervention. Conversely, these results also demonstrate the unexpected ability of adult Chinook salmon to migrate successfully through surprisingly warm temperatures and endure acute thermal stress if sufficiently large volumes of cold water await them at their destination.

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