Last summer, across southwest Alaska’s Bristol Bay region—home to the largest sockeye salmon fishery in the world—tens of thousands of fish washed up dead along riverbanks. Rivers running at temperatures above the threshold for salmon health were killing the fish even as record numbers of them were returning from the ocean to reproduce.
On the Ugashik River, a wide, muddy tributary of the bay, salmon schooled near the river’s mouth, hunkered down in the deeper, cooler water, but they refused to swim upstream into the too-warm waterway. Because no salmon were reaching spawning grounds upriver, the state closed commercial fishing on the Ugashik in early July, right at the normal peak of the run.
Unable to wet their nets and unsure when the fishery would reopen, Ugashik fishermen bided their time at seasonal camps, looking on as jumpers pocked the water all day long. “You’re pretty much watching your income go by,” Catie Bursch, a commercial setnetter on the Ugashik, said later. As Bristol Bay fishermen gear up for this year’s salmon season—one beset by fears that Covid-19 could overwhelm this remote region as thousands of seasonal workers from across the world descend on fishing communities with scant medical resources—they must also contend with a slower-moving hazard: the warming temperatures that threaten a $1.5 billion industry and the people it supports. . .
The state stipulates that water temperature must not exceed 59 degrees Fahrenheit in order for salmon to stay healthy during upstream migration. Last summer, however, river temperatures in Bristol Bay reached 76 degrees. That spells problems for the fish: When salmon can’t avoid warm water, they can sicken or die. Warm water adds stress at a time when fish are already tackling the herculean task of returning to headwater lakes and streams to spawn, making them more susceptible to diseases and speeding up their already-taxed metabolisms. Something like a heart attack can follow: Warm water holds less oxygen than cooler water, but at higher temperatures, salmon actually need more oxygen to survive. Under those conditions, their hearts can’t pump blood fast enough to support their brains and bodies. (Read more from “In Alaska, Summer’s Getting Too Hot for the Salmon Run” HERE)