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

Abstract

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.

Diverse portfolios: Investing in tributaries for restoration of large river fishes in the Anthropocene

“…Further, groups of tributaries may provide unique temperature or river discharge regimes that are distinct from the main channel. The spatial configuration of tributaries may aid in stabilizing mainstem fish abundance by offering contrasting thermal regimes that provide optimal temperatures for juvenile rearing (Phillis et al, 2018), serve as refuge for cool-coldwater fluvial species during warm periods (Barrett and Armstrong, 2022;Tingley et al, 2022), offer thermal refugia habitats necessary for overwintering (Moore et al, 2022a), or support reproduction when suitable temperatures are absent downstream of hypolimneticrelease dams (Clarkson and Childs, 2000). Tributaries can also provide refuge from interspecific competition with invasive species that reside in the mainstem (Pfauserova et al, 2021;Yackulic et al, 2021).…”

Closing the gap between science and management of cold‐water refuges in rivers and streams

“…Here, we focus on recent advances in thermal biology relating to stress, heat tolerance, and CWR use. We also emphasize that various CWPs can serve as CWRs, but their refuge function for thermally stressed organisms can be difficult to assess (Barrett & Armstrong, 2022). In the context of fish thermal requirements, research has focused on thermal performance curves and thermal tolerances.…”

Contrasting long‐term trends in juvenile abundance of a widespread cold‐water salmonid along a latitudinal gradient: effects of climate, stream size and migration strategy

“…Where the integrity and connectivity within, or among, river networks are maintained, some species can make compensatory movements to cooler locations (e.g. tributaries), and establishment of persistent populations can occur if habitat conditions are suitable (Troia et al 2019, Barret and Armstrong 2022). However, anthropogenic barriers (e.g.…”