In the video, the sockeye salmon can be seen with dramatic red lesions and a white fungus that appears when salmon become stressed from overheating.
Conservation group Columbia Riverkeeper recorded the disturbing sight after a massive heatwave in the northwest of the United States.
"Sockeye are dying right now because the Columbia and Snake rivers are too hot,” said Brett VandenHeuvel from Columbia Riverkeeper.
The group claims dams create unnaturally hot water due to huge stagnant reservoirs, and now climate change is pushing it over the edge.
“I’m hopeful this tragedy will inspire our elected leaders to take action to restore our rivers before it is too late,” Mr VandenHeuvel said.
Columbia River reaches 'lethal' temperatures in heatwave
The Columbia River temperature exceeded 21 degrees on the day of the recording, which is lethal for freshwater fish if they are exposed to it for long periods.
The situation has been compared to a person trying to run a marathon in over 38-degree temperatures.
It’s too early to say exactly how many salmon have died as a result of the hot water. But there are tens of thousands of sockeye still in the Columbia and Lower Snake rivers which could be impacted.
There’s fears sockeye salmon will become extinct, with scientists predicting fish kills like this will become more common as climate change worsens.
“The sockeye here are dying,” said Don Sampson from Northwest Tribal Salmon Alliance.
“They are suffocating. You can see they are in lethally hot water. We’re in a salmon crisis.”
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