Glacier retreat creating new Pacific salmon habitat in western North America

Climate change is significantly altering Earth's ecosystems, creating both challenges and opportunities in resource management. Pacific salmon (Oncorhynchus spp.), a species of immense cultural and economic importance, are particularly vulnerable. Declining populations are facing stresses from ocean heat waves, low freshwater flows, and excessively warm water temperatures. However, the warming of Arctic and subarctic freshwaters, coupled with glacier retreat, is creating potential new habitats. While glacier retreat can have various direct and indirect impacts on salmon ecosystems, it also has the potential to generate new streams suitable for colonization. Previous studies have documented regional colonization of recently deglaciated streams, but regional projections for new salmon habitat creation due to glacier retreat remain lacking. This is crucial because glacier retreat also creates opportunities for other industries, potentially leading to conflicts over resource use. Therefore, understanding how glacier dynamics will impact salmon habitats across the Pacific Mountain ranges of western North America is essential for informed management and conservation planning. This study aims to quantify the potential gains in salmon habitat from glacier retreat across this vast region.

The existing literature highlights the vulnerability of Pacific salmon to climate change, documenting negative impacts on various populations due to reduced streamflow, warming temperatures, and ocean heatwaves. Studies also showcase the potential for glacier retreat to create new habitats, with examples of successful salmon colonization in newly formed stream systems. However, previous research primarily focused on localized case studies, lacking comprehensive regional projections for future habitat creation. This study builds on these findings by providing a large-scale assessment integrating glacier mass change projections and habitat suitability models to predict future salmon habitat changes across a vast geographic area.

The study utilized a spatially explicit modeling approach to project future gains in Pacific salmon freshwater habitat. It integrated a model of glacier mass change for 315 glaciers, forced by five different Global Climate Models (GCMs) under Representative Concentration Pathways (RCPs) 4.5 and 8.5, with a simple model of salmon stream habitat potential throughout the Pacific Mountain ranges of western North America. Digital Elevation Models (DEMs) within a Geographic Information Systems (GIS) framework were used to derive a synthetic stream network for glaciated watersheds. Stream gradient thresholds were applied to identify streams accessible to salmon for both spawning and juvenile rearing. The model considered two key life stages: spawning and juvenile rearing, with stream gradient thresholds of 10% and 15% used, respectively. Uncertainty in GCM projections, ice thickness estimates, and stream segment length was propagated through the analysis. The study area encompassed a large region from southeastern British Columbia to southern coastal Alaska, divided into 18 sub-regions. Validation of stream gradient thresholds was done using data from the Suwannee River watershed. Glacier outlines were obtained from the Randolph Glacier Inventory. The model projected the extent of new salmon-accessible streams for the years 2050 and 2100 and estimated the portion suitable for spawning and juvenile rearing habitat.

The study projected that by 2100, glacier retreat will create 6,146 (±1,619) km of new streams accessible for colonization by Pacific salmon. Of this, 1,930 (±569) km have the potential to be used for spawning and juvenile rearing, representing 0 to 27% gains within the 18 sub-regions studied. The timing of new stream exposure varied regionally, with some areas experiencing most gains by 2050 and others showing substantial gains only by 2100. The RCP8.5 scenario, representing higher emissions, projected faster rates of glacier retreat and earlier creation of salmon-accessible streams. Analysis of spawning and juvenile rearing habitat indicated substantial increases, particularly in the Gulf of Alaska and Copper River sub-regions. Projections showed variability depending on watershed topography and salmon species' swimming abilities. The potential for increased salmon production was considerable, with 1 km of suitable habitat potentially producing 500–1500 juvenile sockeye salmon. Inaccessible, high-elevation glaciers are also impacting downstream habitat quality, with declines in meltwater potentially decreasing habitat quality in southern and interior regions but increases in meltwater leading to potentially increased turbidity and cold water in southeastern Alaskan regions like the Copper River.

The findings demonstrate the significant potential for glacier retreat to create new salmon habitat, counteracting some of the negative impacts of climate change. The substantial projected increase in suitable habitat, particularly in certain regions, highlights the importance of considering these gains in management and conservation strategies. However, the study also emphasizes regional variability in the timing and extent of habitat creation. The interplay between gains in new habitat and losses in existing habitat due to altered flow regimes and temperature increases needs further investigation. The study's findings can inform proactive management decisions by identifying areas where new habitats are likely to emerge, allowing for targeted conservation efforts and potentially mitigating conflicts with other resource extraction industries. Further research is needed to better understand the ecological consequences of changes in water temperature, turbidity, and sediment supply in newly created habitats.

This study provides the first large-scale, spatially explicit assessment of potential future gains in Pacific salmon freshwater habitat resulting from glacier retreat. The projected creation of thousands of kilometers of new streams, a significant portion of which is suitable for spawning and rearing, highlights a potentially important positive consequence of climate change for salmon populations in specific regions. The regional variability in habitat creation underscores the need for adaptive management strategies tailored to specific locations. Future research should focus on integrating detailed ecological models to assess the actual productivity of these new habitats and on considering the complex interactions between habitat creation, habitat degradation, and other climate change impacts.

The study acknowledges several limitations. The model's accuracy depends on the reliability of GCM projections and glacier mass balance models. The simplified habitat suitability model may not fully capture the complexities of salmon habitat requirements, and the assumption of a fixed stream gradient threshold for migration may not fully account for species-specific variations or the influence of other factors such as water temperature and flow. Additionally, the study does not consider potential negative impacts of glacier retreat on downstream habitat, such as altered flow regimes or increased sediment loads. Further research is needed to validate the model's predictions and to incorporate additional ecological factors into future assessments.