The cold‐water climate shield: delineating refugia for preserving salmonid fishes through the 21st century

Abstract

The distribution and future fate of ectothermic organisms in a warming world will be dictated by thermalscapes across landscapes. That is particularly true for stream fishes and cold-water species like trout, salmon, and char that are already constrained to high elevations and latitudes. The extreme climates in those environments also preclude invasions by most non-native species, so identifying especially cold habitats capable of absorbing future climate change while still supporting native populations would highlight important refugia. By coupling crowd-sourced biological datasets with high-resolution stream temperature scenarios, we delineate network refugia across >250 000 stream km in the Northern Rocky Mountains for two native salmonids-bull trout (BT) and cutthroat trout (CT). Under both moderate and extreme climate change scenarios, refugia with high probabilities of trout population occupancy (>0.9) were predicted to exist (33-68 BT refugia; 917-1425 CT refugia). Most refugia are on public lands (>90%) where few currently have protected status in National Parks or Wilderness Areas (<15%). Forecasts of refuge locations could enable protection of key watersheds and provide a foundation for climate smart planning of conservation networks. Using cold water as a 'climate shield' is generalizable to other species and geographic areas because it has a strong physiological basis, relies on nationally available geospatial data, and mines existing biological datasets. Importantly, the approach creates a framework to integrate data contributed by many individuals and resource agencies, and a process that strengthens the collaborative and social networks needed to preserve many cold-water fish populations through the 21st century.

Slow climate velocities of mountain streams portend their role as refugia for cold-water biodiversity

“…How much hope ultimately depends on the local biophysical context (e.g., whether a nonnative species is locally abundant or whether natal and adult habitats of migratory taxa are widely separated) and the nexus among how much habitat currently exists, how fast it is changing, and how much is needed by populations to persist. From a solely thermal perspective, however, isotherms are shifting slowly enough that many populations should be able to track them where barriers are not an impediment, and in some cases, unsuitably cold habitats may exist further upstream to offset losses in warmer areas (19,35). Moreover, although climate warming will further isolate populations in headwaters, many organisms that inhabit mountain streams are already adapted to an insular existence and could continue to persist for long periods (36).…”

“…Mountain stream communities evolved with dynamic habitats (43), but populations confronted by these changes are likely to require larger habitats to persist than has historically been the case (44). Estimating those habitat sizes and locations that exceed minimum thresholds as temperatures and other factors change simultaneously will be key challenges in the years ahead (19).…”

“…Many thousands of kilometers of habitat remain uninventoried even for the most charismatic species like salmonid fishes in a well-studied region such as the northwestern United States (19). Moreover, new cold-water species have recently been described in that region (45), which is indicative of the broader appreciation of cryptic biodiversity that is emerging for mountain streams (46,47).…”

“…However, those extreme conditions also make mountain streams resistant to nonnative species invasions such that many indigenous communities remain intact even after a century of climate change and the global pandemic of anthropogenic species introductions (16). As a result, a common pattern is that warmer, low-elevation stream communities are broadly and repeatedly invaded (14,17) whereas colder, high-elevation streams are less altered and often serve as refugia for native species tolerant of cold temperatures (18,19). The same is true regarding more subtle genetic invasions and introgressive hybridization, because many genetically pure populations exist in headwater streams where climatic conditions limit the upstream expansion of hybrid zones (20,21).…”

“…The ubiquity of isolated populations of many species in mountain headwaters (14,18,19) suggests they have existed for extended periods, but climate-induced stream warming (22,23) raises legitimate concerns about their future persistence. Concerns are heightened by evidence of faster air temperature increases at high elevations (24), and by model predictions that streams in snowmelt-dominated landscapes will be highly sensitive (i.e., large stream Δ°C/air Δ°C) to air temperature increases (25)(26)(27).…”