Migration Strategies of Adult Chinook Salmon Runs in Response to Diverse Environmental Conditions in the Klamath River Basin

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.