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Glacial loss and threatened fish: The future of Mount Rainier's cold-water Bull Trout habitats

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Author(s): Kathleen C. Ewen

Category: BOOK
Document Type: M.S. Thesis
Publisher: Western Washington University
Published Year: 2023
Pages: 83
DOI Identifier:
ISBN Identifier:
Keywords: glaciers bull trout Mount Rainier glacial loss climate change watersheds

Glaciers play a key ecological role in the river systems that they support. Cold-water reaches supplied by glacial ice serve as critical habitats for aquatic organisms that rely on specific thermal ranges to survive. Federally threatened Bull Trout (Salvelinus confluentus) require very cold temperatures, like those found in glacial systems, to complete their life cycles. However, glaciers are retreating due to climate change and are expected to continue diminishing throughout this century. Decreased glacial extent could result in warmer stream temperatures downstream from glaciers and, depending on the magnitude of stream temperature increase, cold-water habitats relied upon by Bull Trout and other sensitive species could shrink. This issue is particularly relevant to Mount Rainier (Washington State, USA). Mount Rainier’s dense concentration of glaciers supports several rivers that provide crucial cold-water spawning habitats for Bull Trout. Future scenarios in which Bull Trout spawning habitats are impacted by glacial decline resulting from increased air temperatures have yet to be widely studied on Mount Rainier.

To explore the future of Mount Rainier’s cold-water habitats, I used hourly stream temperature data collected in the glacially-fed White River and Carbon River watersheds, designated as critical Bull Trout spawning habitat by the Endangered Species Act, from June – October in 2021. Based on these empirical stream temperature data, I fit spatial stream network models to each watershed, representing contemporary thermal conditions as a function of current glacial extent and air temperature. Using seven-day average daily maximum (7DADM) stream temperature as my thermal metric and September as my time frame, I focused predictions during Bull Trout spawning season in the White and Carbon rivers. To then simulate future climate change impacts to spawning habitats, I adjusted the models to predict stream temperature in both mid-century and late-century scenarios of air temperature rise, coupled with 20%, 40%, and 80% declines in glacial extent. The average 7DADM temperature predicted for contemporary conditions was 6.3°C in the White River watershed and 8.1°C in the Carbon. As air temperature values increased and glacial size decreased, stream temperatures increased to a maximum of 15.7°C (an increase of 9.4°C) in the White River watershed and up to 12.7°C (an increase of 4.6°C) in the Carbon. The proportion of river kilometers that may be thermally viable for Bull Trout spawning, classified as 12°C, significantly declined in both watersheds by late-century. Site-specific thermal predictions for individual spawning streams found that a few streams may provide cold-water habitats in the coming decades, while most will likely warm beyond a spawning thermal threshold. These results can be utilized by resource managers seeking to conserve Bull Trout and protect the most critical, enduring cold-water habitats. My models can furthermore be used as baselines for future modeling efforts in these or similar glacial systems.

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Suggested Citations:
In Text Citation:
Ewen (2023) or (Ewen, 2023)

References Citation:
Ewen, K.C., 2023, Glacial loss and threatened fish: The future of Mount Rainier's cold-water Bull Trout habitats: M.S. Thesis, Western Washington University, 83 p..