High temperature events shape the broadscale distribution of juvenile Atlantic salmon (Salmo salar)

Abstract

Summer water temperatures within many temperate rivers regularly surpass the incipient lethal temperature for juvenile Atlantic salmon (c. 27°C), causing widescale abandonment of territory in favour of areas of cooler water (thermal refuges). This study aims to highlight the influence of thermal refuges on river‐scale abundance patterns. That is, do salmon parr adjust their distribution over time according to proximity to thermal refuges? Twelve reaches (seven reference: five refuge) were chosen along a 17‐km section of the Little Southwest Miramichi River in Canada. Reaches were sampled throughout the 2011 and 2012 summer periods; high temperature events were recorded during summer 2012 but not summer 2011. Multivariate principal component analyses indicated no discernible difference in habitat characteristics between the reach‐types under normal thermal conditions. However, reaches containing a thermal refuge had a significant increase in relative abundance of parr immediately after a series of high temperature events (water temperature >26°C) in 2012 (p = 0.034). This increase in relative abundance in refuge reaches was not present during the summer of 2011 when no temperature events occurred (p = 0.088), prior to the event of 2012 (p = 0.999), or at the late autumn survey following the 2012 event (p = 0.999). Difference in temperature between refuge and mainstem reaches significantly influenced the suitability of a tributary as a thermal refuge habitat (R2 = 0.84), with preference shown for cooler refuges. River‐wide thermal heterogeneity therefore plays a critical role in survival of juvenile salmon throughout summer months and is likely to become necessary under future climate change scenarios.

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

“…We also found evidence that less variable and cooler temperature regimes in the nearby main stem river led to higher estimated trout abundances within thermal refuges, highlighting the complex roles that main stem rivers play in determining refuge use (e.g., Brewitt et al, 2017;Corey et al, 2023). Railsback and Harvey (2023) expanded on this, implying that any thermal refuge and nearby river characteristics providing opportunities for food intake, lower energy costs, or reduced predation risks are also important.…”

“…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).…”

“…While groundwater is the dominant source for thermal refuges, heavily shaded tributaries or stream channels may also serve as adequate refuge (Dugdale et al, 2013; Ebersole et al, 2003a). To be effective, thermal refuges must be co‐located with the distribution and timing of thermally stressed biota within riverscapes (Baird & Krueger, 2003; Corey et al, 2023). Thermal refuges will, thus, vary widely in habitat and physical conditions both within the refuge and in nearby areas over the short term, which can lead to differing refuge use outcomes (Sullivan et al, 2021).…”

The Role of Cold-Water Thermal Refuges for Stream Salmonids in a Changing Climate—Experiences from Atlantic Canada

“…Corey et al [62] observed that a very high proportion (96-100%) of juvenile Atlantic salmon remained in their original tagging reaches post-thermal events if they contained thermal refuges, but a lower proportion (62%) returned to a reach where no cold-water refuges were located. This finding led Corey et al [120] to investigate the long-term effects of cold-water refuge availability on the relative abundance of juvenile Atlantic salmon along a 17-km segment of river and they found that while the relative abundance increased in the reaches with thermal refuges in summer, an equilibrium in the abundance was regained later in autumn when subsequent thermal events became unlikely. In the same study, Corey et al [120] revealed that the tributary confluence thermal refuges with the highest temperature difference from the main river had the highest effect on the relative abundance of Atlantic salmon across five thermal refuges during the summer season.…”

“…This finding led Corey et al [120] to investigate the long-term effects of cold-water refuge availability on the relative abundance of juvenile Atlantic salmon along a 17-km segment of river and they found that while the relative abundance increased in the reaches with thermal refuges in summer, an equilibrium in the abundance was regained later in autumn when subsequent thermal events became unlikely. In the same study, Corey et al [120] revealed that the tributary confluence thermal refuges with the highest temperature difference from the main river had the highest effect on the relative abundance of Atlantic salmon across five thermal refuges during the summer season. Similar observations regarding a preference towards a larger temperature differential between the mainstem and tributary has also been seen for juvenile rainbow (steelhead) trout [100].…”