Why sockeye flourish and chinook fail in Alaska’s changing climate

University of Washington ecologist Daniel Schindler is at the mouth of a salmon stream at Lake Nerka, in Southwest Alaska. It’s roiling with fish. 

“They sort of pile up in balls of thousands of fish for a couple of weeks. I think that’s when they’re doing their final maturation,” he said of the sockeye mob. “They’re jostling with each other and splashing, occasionally jumping.”

Schindler is in his 27th year of field work, studying Bristol Bay sockeye. This year is on par with the sockeye abundance Bristol Bay has seen in the last decade, he said, which is far higher than the historical average. 

The unlikely hero of this story of plenty: Climate change.

“We tend to think of climate warming is bad news for wild animals,” he said. “But for sockeye Bristol Bay warming has been good news.”

For other salmon, climate change is a villain.

Chinook – or king – salmon are in terrible decline all over the state, and especially dire on the Yukon River. Meanwhile, sockeye – or reds – are having another banner year in Bristol Bay, and everywhere.  Scientists say they don’t know exactly why one salmon species is doing so well while the other is in crisis, but some clues are coming into sharper focus.

One key difference, Schindler said, is what kind of river habit each species needs.

Sockeye use lakes as their nurseries. Since the 1980s the water in those lakes has warmed significantly. The warmth stimulates plankton to reproduce more, and young sockeye eat plankton. Fifty years ago, Schindler said, a lot of sockeye spent two years in Lake Nerka before heading out to sea.

“And now they grow so fast that nearly all of them leave after a single year in freshwater, which is a reflection of the fact that the freshwater systems have become more productive,” he said.

The science is a little murkier about what happens in the ocean, but Schindler said northern parts of the coastal ocean have been especially good for Alaska sockeye. There’s apparently plenty for them to eat and their predators seem to be elsewhere.

“So the Nushagak, the Igushik, even the Kuskokwim River, which never really had that many sockeye in it – all those populations have really exploded in the last decade,” he said.

The chinook aren’t so lucky. Changes in the ocean and the rivers have not been kind to kings, especially for those from Alaska’s longest river, the Yukon.

“It’s kind of this perfect storm of bad things happening for those particular chinook stocks,” said Katie Howard, a state fisheries scientist. 

Her research shows Yukon chinook who spawn during a warm-water year produce fewer juveniles. The water temperatures in the Yukon sometimes get to 68 degrees now. 

“When water temperatures get that high, they just kind of shut down,” she said. “They’re a cold-water fish. They can’t really tolerate those temperatures very well.”

Heat stress is just one factor. Big rainfall can wash eggs from the gravel where female chinooks deposit them. There’s a parasite that leaves Yukon chinook riddled with “pus pockets,” Howard said. And there’s evidence that female chinooks may not be getting enough thiamine from their ocean diet, causing developmental problems in their eggs. 

All of these things may stem from climate change, and kings are particularly vulnerable.

“Kings tend to spawn in really big rivers. That’s where the big king populations are,” said Erik Shoen, a fisheries biologist at the University of Alaska Fairbanks. The Yukon, for instance, has all kinds of conditions along its 1,982 miles, but every fish that spawns there has to go through the lower river.

“So if that lower main stem is unfavorable,” he said, “or if the Bering Sea just went through a heatwave and they have to make it into the lower main stem with less gas in the tank than they need to swim 1,000 miles plus — they’re in trouble.”

By comparison, the sockeye population of Bristol Bay thrive in the ocean and have multiple shorter rivers to climb, with more cool spots to take refuge in.

The Kuskokwim, like the Yukon, is a big river enduring a multi-year crash of chinook. Chum salmon are also in crisis. But there are more sockeye returning to it than ever before.

Near the peak of the Kuskokwim run “there will be anywhere from 20,000 to 40,000 sockeye salmon passing the sonar in one day,” said Kevin Whitworth, executive director of the Kuskokwim River Inter-Tribal Fish Commission. “That’s a lot of protein.”

His organization is encouraging subsistence fishermen to take up dipnets to scoop up sockeye without hurting Kuskokwim chinook. The giant nets – sometimes 5 feet in diameter – are not a traditional tool for the region. 

As part of the campaign, the fish commission posted a video on Facebook featuring testimonials from tribal elders.

“I wasn’t really expecting to get this much from dipping on the Kuskokwim,” said James Nicori, of Kwethluk. “Something new for me. And it works good.”

His brother-in-law, Martin Andrew, also from Kwethluk, said he overcame his skepticism by landing 20 sockeye.

Unfortunately, though, people on rivers like the Kuskokwim can’t just swap one salmon species for another. There still aren’t enough reds returning to replace the missing stocks on the Yukon and Kuskokwim. 

And biologists say there never will be. The Kuskokwim and Yukon just don’t have enough suitable sockeye habitat to produce fish equal to the mass of salmon that used to return to them.

But with chinook too few to meet the need, sockeye are too plentiful to ignore.