Move, migrate, or tolerate: Quantifying three tactics for cold‐water fish coping with warm summers in a large river

Mainstem rivers are important for provisioning cold‐water fisheries and often support fluvial life histories that grow much larger than resident counterparts in tributaries. However, mainstem rivers are also warmer than tributaries during summer and more tightly coupled to regional climate. A key challenge is to understand how fluvial life histories cope with summer temperatures in mainstem rivers, both now and in a warmer future. Here, we outline three options: tolerance, exploitation of cool microhabitats, or migration to cooler tributaries. We quantified the relative prevalence of these responses in the wild using a Lagrangian approach. Specifically, we used temperature transmitting radio‐tags to track coastal cutthroat trout ( Oncorhynchus clarkii clarkii ) habitat use across 100 km of the Willamette River, which exhibited a maximum temperature of 22°C. We found that 89.8% of fish stayed in the mainstem and tolerated high temperatures; 6.5% of the population exploited cool floodplain alcoves; and 3.7% moved into a cool tributary, the McKenzie River. We also found that on average, larger fish opted to use cold‐water refuge habitats rather than tolerate mainstem river temperatures. The expression of all three coping tactics suggests this population has adaptive capacity to respond to future warming. More work is needed to understand how the relative expression of these tactics change depending on the severity of heat stress or nontemperature attributes of mainstem, floodplain, and tributary habitat.