Winter survival and habitat as limiting factors for Arctic grayling at Red Rock Lakes National Wildlife Refuge

• Main Author: Davis, Michael Nordstrom
• Other Authors: Chairperson, Graduate Committee: Thomas E. McMahon, Thomas McMahon was a co-author of the article, 'Factors affecting winter hypoxia in a large boreal lake: evidence for a hypoxia refuge and for reoxygenation prior to spring ice loss' submitted to the journal 'Transactions of the American Fisheries Society' which is contained within this thesis., Thomas McMahon and Molly Webb were co-authors of the article, 'Winter survival, habitat use and hypoxia tolerance of Arctic grayling in an ice-covered boreal lake prone to winterkill' submitted to the journal 'Transactions of the American Fisheries Society' which is contained within this thesis.
• Format: Thesis
• Language: English
• Published: Montana State University - Bozeman, College of Letters & Science 2016
• Subjects: Arctic grayling; Winter; Habitat (Ecology); Arctic; Red Rock; Rock Lake; Thymallus arcticus
• Online Access: https://scholarworks.montana.edu/xmlui/handle/1/9766

Arctic grayling (Thymallus arcticus) are a now rare, glacial relict species occupying only a fraction of their historic range in Montana. The population in Upper Red Rock Lake is the only significant remaining native, adfluvial population and has undergone significant declines in abundance and distribution. Previous studies have documented instances of very low winter DO in the lake and low overwinter survival due to winter hypoxia has been hypothesized as a potential limiting factor for this population. We tested this hypothesis using a combined laboratory and telemetry study to document extent of hypoxia in Upper Red Rock Lake over two winters and assess the physiological tolerance, behavioral response and winter survival rate to hypoxia. In the laboratory study we observed a significant behavioral and physiological response to DO < 4 mg/L and determined 24-hr LD 50 values of 0.75 mg/L for adults and 1.50 - 1.96 for juveniles at temperatures of 1 - 3°C. In the field study we recorded a period of significantly lower lakewide average DO as well as high spatial variability in DO concentration (< 1 - 10 mg/L) during ice cover. However, we found that 69 - 100% of the lake epilimnion had DO > 4 mg/L during the ice cover period of both winters. Results of the telemetry study indicated adult winter survival rate was high (0.97 in 2014, 0.95 in 2015) and that telemetered fish selected deeper, more oxygenated habitat during ice cover. Our study demonstrates that Arctic grayling have a very high tolerance to acute hypoxia exposure and exhibit a physiological and behavioral stress response to DO < 4 mg/L. Although hypoxia was present in Upper Red Rock Lake, ample suitable habitat was available in the epilimnion in both study winters. Despite the potential for a hypoxia threat to develop in more severe winters, we conclude that low winter survival due to winter hypoxia is likely not a limiting factor for this population.