Upper Thermal Limits to Migration in Adult Chinook Salmon: Evidence from the Klamath River Basin

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 were approximately 23°C on average (22.9°C) with a range of 21.8 to 24.0°C. During the first week (168 h) of migration, mean average body temperature was approximately 22°C (21.9°C), mean average minimum daily body temperature was approximately 21°C (20.6°C), and mean average maximum daily body temperature was approximately 23°C (23.1°C). 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.