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I stood on the deck of a fishing boat this past summer, soaked from sweating inside my rain gear after an hour of picking sockeye salmon from our gill net at the mouth of the Nushagak River on Alaska’s Bristol Bay. Fishing is hard enough work, but an early July heatwave sizzled the region, making for grueling conditions on deck. The temperature in southwest Alaska pushed 32 °C. Smoke blowing in from wildfires burning hundreds of kilometers east blotted out the mountains on the northern horizon. Nothing about the conditions was normal.

I’ve worked as both a journalist and a commercial fisherman for over a decade, participating in more than a dozen fisheries from Southern California to the western Gulf of Alaska. I’ve seen booms and busts over the years, and this summer the fishing in Bristol Bay was booming. Estimates say 56.3 million salmon returned to the bay’s rivers. While down from 2018’s record-breaking runs, with 62.3 million fish, Bristol Bay has so far bucked the trend of declining salmon runs seen in other regions. But all is not well. As I was sweating on deck, the water was 18.9 °C—just a few degrees shy of 21 °C, when the temperature starts being lethal to salmon.

Twenty-five kilometers northwest, in the nearby Igushik River, the water was even warmer. One hundred thousand sockeye salmon waited for cooler conditions so they could move upstream to spawn. But, unwilling to pass through the hot, shallow water, the fish used up the available oxygen and suffocated—it was the largest sockeye salmon die-off seen in Bristol Bay, says Timothy Sands, a biologist with the Alaska Department of Fish and Game. Elsewhere in the watershed, temperatures also soared.

On July 4, Anchorage hit 32.2 °C for the first time in recorded history. Six days later, the Cook Inletkeeper, a nonprofit organization based in nearby Homer, announced that salmon stream temperatures were “off the charts,” with water in the Deshka River, northwest of Anchorage, reaching 27.6 °C. In Norton Sound, there was a reported mass die-off of pink salmon. Up the Koyukuk River, a tributary of the Yukon River, there was another heat-related die-off, this time of chum salmon that had yet to spawn.

“I have never seen a summer as extreme as this year,” says Sands, who has worked in the Bristol Bay area since 2002. By summer’s end, Sands and his colleagues saw dead fish in every river in Bristol Bay.

The Pebble Mine, a massive open pit gold and copper mine proposed for the headwaters of Bristol Bay, poses a great threat to the world’s largest sockeye salmon run. But a rapidly changing climate and increasingly warm water could be an even bigger problem for the largely untouched salmon grounds. It’s a threat that can seem difficult to reconcile with the recent record-breaking seasons.

“Up until now, [salmon] runs have been exceedingly strong,” says Thomas Quinn, an ecologist at the University of Washington who has spent almost every summer since 1987 studying salmon in Bristol Bay. “Most would say there’s clearly not a problem.”

While the nitty-gritty of salmon’s response to climate change “is more complicated than rocket science,” says Quinn, for both him and Sands one thing is clear: warmer water contributes to faster growth for individual fish and their populations as seen in the huge salmon runs in recent years—up until the lethal threshold.

“This summer baked Alaska. This summer we entered new territory with these unprecedented temperatures,” Quinn says. “But it will take four or five years before we’ll know the effects.”

Across the scientific community the consensus is that even if we decarbonize the global economy immediately, the effects of a century of greenhouse gas emissions will still lead to increasing temperatures for decades. These mass die-offs are likely a harbinger of things to come, says Quinn.

Bristol Bay fishermen have been united in their stand against the Pebble Mine, becoming unlikely environmentalists as they have built coalitions with a diverse group of allies locally and across the United States. But the fight for salmon is bigger than Bristol Bay and the Pebble Mine.

For Quinn, the climate impacts we saw this summer in Alaska go beyond sustainable fish.

“It’s easy for fishermen to see a gold mine is bad, but these broader environmental issues are threats as well,” Quinn says. “But quite frankly, if this rate of change continues Bristol Bay sockeye will be the least of our worries.”