Winter Survival, Habitat Use, and Hypoxia Tolerance of Montana Arctic Grayling in a Winterkill‐Prone Lake

Winter hypoxia in shallow, ice‐covered lakes can be a significant limiting factor for overwintering fish populations. In this study we tested the hypothesis that low overwinter survival due to winter hypoxia is a limiting factor for a rare, adfluvial population of native Arctic Grayling Thymallus arcticus inhabiting Upper Red Rock Lake, Montana. We used a combined laboratory and telemetry study to document the extent of hypoxia over two winters and to assess the physiological tolerance, behavioral response, and winter survival in relation to hypoxia. In the laboratory, we observed a significant behavioral and physiological response to dissolved oxygen (DO) levels ≤ 4.0 mg/L and determined acute 24‐h LC50 values (concentration lethal to 50% of test fish) of 0.75 mg/L DO for adults and 1.50–1.96 mg/L for juveniles at temperatures of 1–3°C. In the field study, we observed dynamic DO concentrations (DO 4.0 mg/L in the epilimnion. Radiotelemetry indicated adult winter survival rate was high (0.97 in 2014, 0.95 in 2015) and that telemetered fish selected deeper (>1 m), more oxygenated habitat during ice cover. Our study demonstrated that Arctic Grayling have a high tolerance to acute hypoxia exposure and exhibit a physiological and behavioral stress response to DO concentrations ≤ 4.0 mg/L. Although hypoxia was present in parts of the lake, sufficient suitable habitat with DO > 4.0 mg/L was available in the lake epilimnion in both study winters. However, winter conditions more severe than those observed during our 2‐year study occur periodically in the lake, and thus winterkill could still be a limiting factor for the population.