Warming Rivers Are Causing Die-Offs Among Alaska Salmon

Dead chum salmon lined snow-fed and rain-fed rivers across Alaska, where lethally high temperatures and low water levels prevented migration. With their original habitat under threat by global warming, cold glacial water is becoming more necessary for the survival of salmon.

Salmon are born in freshwater rivers. They swim to the ocean to spend their adulthoods, and then return to the rivers to spawn four or five years later, Holly Carroll, a Yukon-focused biologist at Alaska’s Department of Fish and Game, told GlacierHub.

Salmon felt the effects of July’s record-breaking temperatures in Alaska this summer as they struggled to reach their spawning grounds. They do not eat once they begin the journey back to the rivers, Carroll said, meaning that they have a limited amount of energy to make the trip. “So if they then encounter really warm water, it puts their body under much more stress,” she said. The heat speeds up their metabolisms so that salmon can run out of energy and die mid-journey.

As salmon suffer in Alaska’s warming snow- and rain-fed rivers, some scientists are looking to glaciers to predict the future of these fish. 

The Die-Offs

According to NPR, the largest die-off occurred in the Koyukuk River, a tributary of the Yukon River. The dead salmon count was in the thousands to ten thousands, Stephanie Quinn-Davidson, director of the Yukon River Inter-Tribal Fish Commission, told GlacierHub. She put together a team of scientists and surveyed 200 miles of the river to count the salmon—they found at least 850 dead —and confirm, by cutting the fish open, that they had not spawned and had no signs of disease.

“The die-off coincided with record-breaking temperatures in Alaska,” she wrote in a Facebook post.  Some places on the Koyukuk reached 90 degrees Fahrenheit from July 7 to July 11—25 degrees above average. July 12, she noted, was when when locals began seeing dead chum salmon floating downriver.

Quinn-Davidson’s team examines salmon along the Koyukuk River. (Source: Stephanie Quinn-Davidson)

The Koyukuk is fed by snow and rain, Carroll told GlacierHub. This year, the snow melted quicker, leaving the river with record-high water levels earlier in the season and very low levels in the summer. The drought also deprived the river of much-needed rainwater.

The Bristol Bay area also experienced salmon die-offs, particularly in the Igushik River, which flows from Amanka Lake into Bristol Bay. Tens of thousands of fish were found dead in the Igushik, according to Timothy Sands, area management biologist of the Nushagak/Togiak region.

An early-melting snowpack led to low water levels, aiding in the rise of temperatures, Sands told GlacierHub. The snowpack usually continues to melt through June, he said, but this year it vanished in May. Lower water levels enhanced the heating of these waters which are already susceptible to warming as a muddy river in the tundra.

While Koyukuk salmon died of heat stress, the Igushik salmon died of oxygen depletion, Sands said. The river is prone to oxygen depletion due to its geography: The elevation drop in the river is minimal, and so the flow of the river is slow and tidal. This means less recycling of water and less replenishment of oxygen.

High temperatures increase a salmon’s metabolism, so that it increases its need for oxygen at a time when there is less of it in the water, Mary Catherine Martin, communications director for Salmon State, told GlacierHub.

Even with the die-off, Sands said, the river reached its escapement goal, which is the number of fish that are required to reach their  spawning grounds in order to ensure a new generation of fish. “It’s a healthy system,” he said, noting that this year’s run was the third highest since 1884. 

Dead chum salmon lined the Koyukuk River this July. (Source: Stephanie Quinn-Davidson)

Habitat in Danger

Sue Mauger, science director at Cook InletKeeper, has been monitoring stream temperatures in Alaska’s Cook Inlet since 2002. She told GlacierHub that she was surprised there weren’t more salmon die-offs this summer, considering that the waters reached temperatures that are considered lethal for salmon. In the Deshka River, she said, temperatures were warmer than those expected for 2069 under a worst-case climate model.

The fish survive by finding cold-water refugia, which are pockets of cold water in which the salmon can wait for temperatures to diminish. In the case of the Deshka, she said, a connected glacial area provided this space for salmon to wait out the heat.

In deep rivers, groundwater inflows, side channels and springs provide the same service. But when rivers experience low water levels, “you don’t have those deep cold refuges, then the salmon don’t have anywhere to hang out and wait until the temperatures start to drop,” Quinn-Davidson told GlacierHub.

Development also harms this habitat, Mauger told GlacierHub. Abstaining from building near rivers is “a decision that we need to make,” she said.

In Bristol Bay, the Environmental Protection Agency took a step towards the opposite decision this month by allowing a mining project to progress unopposed, despite the risks to salmon.

A New Glacial Habitat

“This summer has a lot of the components that we should expect to see in the future, and that includes having a warm spring, which means that however much snowpack we have is going to melt out earlier,” Mauger said.

Salmon are finding new spawning grounds in the vast habitat of cool streams that Alaska’s receding glaciers are leaving behind. In Glacier Bay, Kenai Fjords National Park, and other areas, salmon are beginning to inhabit these streams that benefit from ice melt all summer long.

But the situation is more complicated, according to Chris Sergeant, a researcher at the University of Alaska, Fairbanks who studies the effects of temperature and other stream conditions on salmon. “While a glacial river might protect salmon from warm water temperatures [in the summer], they may not necessarily be the best place for salmon to grow in rear during all the other months,” he said. In the winter, he told GlacierHub, young salmon grow faster in warmer waters. Since glacial streams are prone to flooding, spawning there also means a risk that the eggs will be swept away in water.

In the summer, he said, “some of these fish will live in the glacial systems for a few months, and then they’ll move into a smaller tributary fed by rain or snow” for the remainder of the year.

These are juvenile fish, though. After salmon spend their adulthoods in the ocean, they return to the exact rivers they were born in to spawn the next generation, for better or worse. “A lot of studies recently suggest that not only do they return to the same river where they were born, but they also spawn… within twenty feet of where they were born,” Martin said.

Glacial mainstem rivers (the brown water) mix with other habitats in the Taku River watershed, located between Alaska and British Columbia. Preserving a diversity of habitats will help ensure healthy salmon populations continue in the face of climate change, Sergeant told GlacierHub. (Source: Chris Sergeant for the University of Alaska Fairbanks and the University of Montana)

Response to This Year’s Die-Offs

There is not much to be done about the die-offs besides collecting data, Carroll said. The locals—many of whom are native Alaskans—are key to this. With extensive tributaries, the Dept. of Fish and Game relies on locals to report natural events in the river.

In the case of the Koyukuk river, Quinn-Davidson said, the expedition she organized would not have happened without the Yukon River Inter-Tribal Fish Commission, which represents over thirty federally recognized tribes along the Yukon River. “Alaska is currently experiencing a pretty major budget crisis,” she said. “It was good that our organization could step in, bring us all together, provide the funding to take us all out there, and more properly document what was going on.”

Salmon are of great importance to the local communities, who rely on the fish for food and their livelihoods. A commercial fishery for chum salmon is located on the lower part of the Koyukuk. “That fishery really helps the local economy because most people that live along the Yukon are native Alaskans and there’s not a lot of local jobs,” she said.

“People were able to meet their needs for salmon this year despite the die-offs,” Quinn-Davidson said. “Now the big question is, how much will this die-off impact future years of salmon returning—because those salmon didn’t end up spawning.”

According to Carroll, it’s likely that the next generation will not be negatively affected, since fewer young salmon can minimize  over-competition.

The die-offs did not pose an existential threat to salmon this year. According to Martin, the multitudes of dead salmon, along with the state’s wildfires, were a reminder that “climate change is happening, and it’s real, and it’s going to be an increasing part of the conversation here in Alaska.”

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