Geologic Publications for Mount Rainier

Exploring thermal occupancy between bull trout and brook trout in glacial headwaters

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Author(s): Nicole G. Ring

Category: PUBLICATION
Document Type: M.S. Thesis
Publisher: Western Washington University
Published Year: 2025
Volume:
Number:
Pages: 61
DOI Identifier:
ISBN Identifier:
Keywords: bull trout brook trout glacially influenced streams glaciers climate change temperature

Abstract:
Cold, glacially influenced headwater streams are recognized as critical thermal refugia for cold-water fish species such as bull trout (Salvelinus confluentus), which are highly sensitive to warming temperatures and face significant threats from climate change and non-native species. The cold waters of glacial streams not only provide essential thermal conditions for bull trout but may also exclude non-native species like brook trout (Salvelinus fontinalis) that compete with or displace bull trout. My study assesses the distribution of bull and brook trout in two glacial headwater rivers within Mount Rainier National Park, an area that includes federally designated critical habitat for bull trout under the U.S. Endangered Species Act. Using environmental DNA (eDNA) sampling and spatial stream network modeling (SSNM), I examined the relationship between fish presence and water temperature to understand thermal occupancy within glacial headwaters. Results showed that bull trout were widely distributed across the study site and detected at temperatures as low as 3 °C. Brook trout were concentrated in the warmest areas of the study area, being most prevalent between 6 and 8 °C, with the probability of their presence increasing as temperatures increased. Brook trout were nonetheless observed at temperatures as low as 4.25°C, which indicates a lower thermal tolerance than previously documented and calls into question the idea that temperature is a limiting factor for their distribution. My findings present evidence that brook trout may be more tolerant of cold stream temperatures than previously thought, and low stream temperatures may not be as strong of a barrier to invasion. Greater understanding of thermal occupancy of bull trout and brook trout in glacial headwaters is necessary to inform management strategies to mitigate the risk of bull trout extirpation in these vulnerable ecosystems.

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Suggested Citations:
In Text Citation:
Ring (2025) or (Ring, 2025)

References Citation:
Ring, N.G., 2025, Exploring thermal occupancy between bull trout and brook trout in glacial headwaters: M.S. Thesis, Western Washington University, 61 p..