Winter Habitat and Survival As Potential Limiting Factors for Arctic Grayling at Red Rock Lakes National Wildlife Refuge, Montana

Arctic grayling at Upper Red Rock Lake, Montana are the only remaining native, lacustrine grayling population in the contiguous United States. Upper Red Rock Lake is a shallow, productive, high elevation lake with high winterkill risk that is hypothesized to limit grayling abundance. Laboratory experiments and radio telemetry were used to determine low oxygen tolerance and winter habitat use and survival. In laboratory trials adult and juvenile Arctic grayling displayed higher than expected tolerance to hypoxia at winter water temperatures, surviving 24 – hr exposures to 1 mg/L and 2 mg/L O₂, respectively. Hypoxic conditions were widespread across deeper depths (>1m) of the lake during ice cover in the winters of 2014 and 2015.  However, the presence of a rather large envelope of shallow, oxygenated (> 4 mg/L) water beneath the ice throughout the winter appeared to offer a refuge from hypoxia. In both winters, overwinter survival of telemetered grayling during lake ice over (Nov-April) was surprisingly high (>90%).  Study results indicate that Arctic grayling have an unusually high tolerance to low dissolved oxygen, but the spatial and temporal extent of the normoxic envelope likely varies from year to year and may limit survival during severe winters.