Cooke, C.; Graydon, J.; Luek, A.; Lu, X.; Yu, H.; Le, X. C.; Reichert, M.

Mountaintop removal (MTR) coal mining generates large volumes of waste rock. Weathering of this waste rock releases selenium, which can bioaccumulate to levels that can harm, and even extirpate, downstream fish communities. This is well demonstrated in ecosystems impacted by active MTR operations; however, less is known about the long-term impacts after coal mines close. Here we show that MTR coal mines still present an acute threat to downstream fish populations, decades after mining ends. Crowsnest Lake (Alberta, Canada) receives runoff from the Tent Mountain Coal Mine, which closed in the 1980s and part of which was certified reclaimed. Fish in Crowsnest Lake contain tissue selenium concentrations (5-26 g/g dry weight) that exceed guidelines and rival fish selenium levels downstream of active MTR operations. This is despite lake water selenium concentrations ([≤]2 g/L) that are below water quality guidelines intended to protect fish. The clinical signs of selenium poisoning in fish are similar to the symptoms of Whirling Disease, which was first detected in the Crowsnest basin in 2016 making this the first aquatic system to be impacted by both stressors. This finding demonstrates that the biological impacts of MTR coal mining can persist long after mining operations end, and it suggests that any further coal mine development may well push the Crowsnest fishery beyond sustainability.