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

Abstract

Objective:The literature on thermal refuges has focused on when fish use what kinds of refuge, but we address how refuge availability and characteristics affect population abundance and persistence under warming.Methods: Population benefits of thermal refuges are very difficult to assess em-

Disentangling effects of habitat on salmonid abundance in thermal refuges while accounting for imperfect detection

“…We estimated a maximum of 118.86 (95% CRI = 1.27–815.93) trout within the Furnace Brook refuge and 2.272 trout m −2 within the Pine Swamp Brook thermal refuge across all surveys. Fish abundance within thermal refuges may vary with population abundance within the nearby main stem river, the availability of thermal refuges throughout the river segment, and main stem river temperatures (e.g., Corey et al, 2023; Ebersole et al, 2001; Railsback & Harvey, 2023), making direct comparisons across studies difficult. Still, our estimates of trout density within thermal refuges were similar to estimates reported in other riverscapes (Brewitt et al, 2017; Morgan & O'Sullivan, 2023; Wang et al, 2020; Wilbur et al, 2020).…”

“…Persistence in marginal riverscapes is, however, possible. Riverscapes that are ephemerally warm during the hotter summer months but suitable during other periods can support cold‐ and cool‐water‐dependent fisheries (Armstrong et al, 2021), suggesting that cold‐ and cool‐water‐dependent fish persistence may only be constrained by available suitable thermal refuge habitats (Railsback & Harvey, 2023). Prolonged exposure to warm water temperatures approaching critical thresholds can improve cold‐water fish acute thermal tolerance and thermal resilience without mortality (e.g., Corey et al, 2017), but the more frequent, prolonged, and extreme heat events will impose physiological consequences (e.g., O'Sullivan et al, 2023).…”

“…Prolonged exposure to warm water temperatures approaching critical thresholds can improve cold‐water fish acute thermal tolerance and thermal resilience without mortality (e.g., Corey et al, 2017), but the more frequent, prolonged, and extreme heat events will impose physiological consequences (e.g., O'Sullivan et al, 2023). Thermal refuges temporarily protect cold‐ and cool‐water‐dependent fish against extreme temperatures for minutes to weeks during heat waves and periods of prolonged high ambient stream temperature, representing opportunities for resistance to heat events (e.g., Railsback & Harvey, 2023). Our hierarchical analysis using a Bayesian N ‐mixture model led to robust abundance estimates and identification of influential covariates, and we demonstrated short‐term variations in abundance during a heat wave due to environmental conditions while accounting for imperfect detection.…”

“…Our finding that the thermal refuge surface area (in square meters) was a stronger driver of trout abundance compared to depth (in meters) or the temperature differential (in degrees Celsius) highlights the roles that larger refuges may play in protecting populations against heat waves. For example, Railsback and Harvey (2023) found that cold‐water‐dependent salmonids can persist in river sections where thermal refuges compose at least 2% of the stream area during a prolonged heat wave. In contrast, we found that trout thermoregulate in small thermal refuges intermittently during a heat wave (Figure 6), perhaps because smaller groundwater‐dominated tributaries tend to be cooler than larger tributaries (e.g., Ebersole et al, 2001).…”

“…Others must rely on localized thermal refuges, or cold‐water patches that fish inhabit for temporary thermal relief, for short‐term buffering against weather extremes (Amat‐Trigo et al, 2023; Brewitt et al, 2017; Kurylyk et al, 2015; Sullivan et al, 2021; Torgersen et al, 2012; Wilbur et al, 2020). Thermal refuges are critical for the short‐term survival of many individuals of a population, which can scale up to influence a species' adaptive capacity and resistance of a cold‐water‐dependent fish community to extreme weather (e.g., Beever et al, 2017; Lynch et al, 2014; Railsback & Harvey, 2023; Thorpe, 1994). Thus, thermal refuges play a critical role in the current and future conservation of cold‐water‐dependent biota in this era of rapid climate change.…”