Exposure Risk of Fish Downstream of a Hydropower Facility to Supersaturated Total Dissolved Gas

Fish exposed to supersaturated total dissolved gas (TDG) levels can develop gas bubble trauma (GBT) which can lead to sublethal effects or mortality. Access to refugia in areas of high TDG that allows for hydrostatic (depth) compensation can mitigate exposure risk and GBT occurrence. The goals for this study were to examine resident fish habitat and depth use and assess exposure risk to elevated TDG levels related to hydropower operations in the Columbia-Kootenay system in British Columbia. Modeling was used to predict TDG levels for three operational cases (low, medium, and high spill rates). Acoustic telemetry was used to track rainbow trout Oncorhynchus mykiss (RT) and mountain whitefish Prosopium williamsoni (MW) reach and depth residency. Telemetry results did not differ among operational scenarios and aligned with known biological/life history characteristics for fluvial or fluvial-adfluvial species. Within-species MW reach residency appeared to be reflective of seasonal habitat selection for spawning, foraging, and refuge movements. Within-species RT reach residency appeared to follow habitat association patterns reflective of RT ecology. A risk assessment revealed that RT had a significantly higher TDG exposure risk relative to MW, but sufficient depth refugia habitat was available to mitigate exposure risk and GBT occurrence in both species. The results suggested that TDG exposure risk and actual risk depend on the interplay between species-specific ecology and TDG patterns generated by hydropower facilities. The ecological and TDG patterns in this study suggested that system- and species-specific studies will be required to generate detailed TDG exposure predictions for management decision-making.