| Summary: | This research investigated a population of lake chub (Couesius plumbeus) inhabiting a geothermal spring complex south of the community of Atlin, in the far northwest of British Columbia. These lake chub live in thermal isolation in 13-26°C water year round, while the only neighbouring water body, glacier-fed Atlin Lake, remains below 6°C year round and has no known population of lake chub. Through mark-recapture sampling I estimated the population size of this isolated and physiologically distinct population to be on the order of 1,000 to 2,000 mature individuals. I measured its area of occupancy as 3,602m². As such, the population is small in both habitat extent and population size, as well as being physically isolated from other populations of this widespread species. Tagging of fish in individual sections of the warm springs showed a general lack of movement among different sites. Size-frequency histograms for individual sites and points in time illustrated the presence of distinct cohorts suggesting lifespans of three to four years, with outliers living a year or two longer. I also studied the invasive cherry shrimp (Neocaridina davidi var. red), performing experiments to observe survivorship and behaviour under different temperature regimes. I found no significant decrease in mortality in adults when water was cooled to 5°C, but a significant reduction in their ability to react to stimuli below 15°C. I also observed 100% mortality in juveniles at water temperatures of 10°C or lower within two days. Based on these data, barring future adaptation to considerably colder conditions, is unlikely that cherry shrimp will be able to expand their range further into the Yukon River basin. Overall, my study provides a baseline of understanding about the population and life history of an isolated warm-springs population of lake chub, and provides observations of the early stages and probable outcomes of an exotic species invading an isolated location. Science, Faculty of Zoology, Department of Graduate
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