Tough places and safe spaces: Can refuges save salmon from a warming climate?

Abstract

The importance of thermal refuges in a rapidly warming world is particularly evident for migratory species, where individuals encounter a wide range of conditions throughout their lives. In this study, we used a spatially explicit, individual‐based simulation model to evaluate the buffering potential of cold‐water thermal refuges for anadromous salmon and trout (Oncorhynchus spp.) migrating upstream through a warm river corridor that can expose individuals to physiologically stressful temperatures. We considered upstream migration in relation to migratory phenotypes that were defined in terms of migration timing, spawn timing, swim speed, and use of cold‐water thermal refuges. Individuals with different migratory phenotypes migrated upstream through riverine corridors with variable availability of cold‐water thermal refuges and mainstem temperatures. Use of cold‐water refuges (CWRs) decreased accumulated sublethal exposures to physiologically stressful temperatures when measured in degree‐days above 20, 21, and 22°C. The availability of CWRs was an order of magnitude more effective in lowering accumulated sublethal exposures under current and future mainstem temperatures for summer steelhead than fall Chinook Salmon. We considered two emergent model outcomes, survival and percent of available energy used, in relation to thermal heterogeneity and migratory phenotype. Mean percent energy loss attributed to future warmer mainstem temperatures was at least two times larger than the difference in energy used in simulations without CWRs for steelhead and salmon. We also found that loss of CWRs reduced the diversity of energy‐conserving migratory phenotypes when we examined the variability in entry timing and travel time outside of CWRs in relation to energy loss. Energy‐conserving phenotypic space contracted by 7%–23% when CWRs were unavailable under the current thermal regime. Our simulations suggest that, while CWRs do not entirely mitigate for stressful thermal exposures in mainstem rivers, these features are important for maintaining a diversity of migration phenotypes. Our study suggests that the maintenance of diverse portfolios of migratory phenotypes and cool‐ and cold‐water refuges might be added to the suite of policies and management actions presently being deployed to improve the likelihood of Pacific salmonid persistence into a future characterized by climate change.

Multi‐scale relationships in thermal limits within and between two cold‐water frog species uncover different trends in physiological vulnerability

“…The possible refuges that we uncovered may not be facilitating behavioural thermoregulation in this system but do underscore an important alternative outcome: isolated or clumped thermal refuges may incur other costs and have the potential to form ecological traps (sensu Battin, 2004). For example, even in spatially heterogeneous streams, organisms do not always choose to exploit thermal refuges (e.g., Barrett et al, 2022), perhaps owing to the costs associated with behaviourally thermoregulating, such as increased predation risk and/or associated bioenergetic costs (e.g., Huey & Slatkin, 1976; Carrascal et al, 2001; Snyder et al, 2022). Studies that incorporate bioenergetic information into spatially‐explicit forecasting models with other components of adaptive capacity (plasticity, evolvability, dispersal) will fill a key gap in our understanding of vulnerability to changing environments.…”

Can thermal refuges save salmonids? Simulation of cold‐pool benefits to salmonid populations

“…The model InSTREAM embodies much of our knowledge of how temperature and other habitat characteristics affect individual salmonids in a framework that makes long‐term population predictions, making simulation studies such as this one a way to explore such important yet difficult questions. The simulation approach has been used before; Snyder et al (2022a, 2022b) used a larger‐scale individual‐based model to evaluate thermal refuge benefits for upstream‐migrating salmonids.…”

“…The model InSTREAM embodies much of our knowledge of how temperature and other habitat characteristics affect individual salmonids in a framework that makes longterm population predictions, making simulation studies such as this one a way to explore such important yet difficult questions. The simulation approach has been used before; Snyder et al (2022aSnyder et al ( , 2022b) used a larger-scale individual-based model to evaluate thermal refuge benefits for upstream-migrating salmonids. Using simulation, as with field studies, we need to consider the limitations of our analysis and evidence for its validity and then look for conclusions of general relevance to understanding and managing fish populations.…”

Cold-water habitats, climate refugia, and their utility for conserving salmonid fishes

“…Because facultative residency is not an option in these circumstances, management actions may have to increasingly focus on maintaining suitable thermal conditions in migratory corridors where warm temperatures are limiting to ensure life cycle completion. Thermal refuges will be vital for temporarily harboring migrants along these corridors, and efforts to describe and inventory cold-water anomalies with TIR surveys (Dugdale et al 2015;Fullerton et al 2015) may need to become more predictive to aid in the design and implementation of projects that create stepping-stone habitats that meet the ecological requirements of target species (Hannah et al 2014;Kurylyk et al 2015;Snyder et al 2022).…”