Scientists know one thing for sure about the collapse of Yukon River king and chum salmon: there’s more than one culprit.
“It’s really hard and probably unrealistic to just point your finger at one thing and say that’s what’s doing it,” said Jayde Ferguson, a fish pathologist with the Alaska Department of Fish and Game.
Researchers have identified many threats facing Yukon king and chum salmon, and those threats pop up at each stage of the salmon life cycle — when salmon hatch in freshwater streams, as they swim down the Yukon to the ocean, where they spend most of their lives and on their arduous journey back upriver to spawn and die.
Scientists think many of these threats are connected to climate change. Ferguson studies one of them, a parasite named ichthyophonus, at the Alaska Department of Fish and Game lab in Anchorage.
Under a microscope, salmon tissue infected with ichthyophonus appears mottled with big white dots, each one a single parasite that will grow, draining the fish’s resources and causing cells to die.
The parasite can’t harm humans, but it does kill fish. As salmon are making their journey upstream, they’re especially vulnerable.
“Their immune system is not as good, their bodies are just breaking down,” Ferguson said. “And so the parasite actually starts replicating then within the fish.”
Many infected fish don’t survive long enough to lay eggs.
“It’s almost like an arms race,” he said. “Can they get to the spawning ground before they die prematurely?”
Often, infected fish look completely normal from the outside, but their flesh will have a spotted or patchy white pattern where the parasite is growing inside. Importantly, infected fish aren’t good to eat.
Researchers saw a big spike in king ichthyophonus levels in the early 2000’s, when around 30% of kings showed detectable levels. Levels dropped off for more than a decade. And then in 2020, the parasite was back. In recent years more than 40% of the Yukon king run has shown detectable levels of ichthyophonus.
It’s unclear what’s driving the spike. Other researchers have found that Yukon king salmon eggs are low in a vitamin called thiamine, which may cause weakened immune systems. Ferguson said warming river water might also play a role.
In fact, the Yukon is warming twice as fast as rivers further south as a result of climate change.
“It’s crazy to be at the northern-range extent of salmon and talking about it being too hot for them,” said Vanessa von Biela, a U.S. Geological Survey ecologist.
Salmon are cold-blooded, meaning they can’t regulate their internal temperature. When the river gets above 65 degrees Fahrenheit, that’s a problem.
When it’s too hot, Von Biela said, the proteins that keep salmon cells functioning normally start to lose their shape. Warm water also makes it harder for their hearts to pump oxygen to their bodies.
“Their whole physiology, their whole body is designed to be in cold water,” she said. “So when that water is warm, they just really hit these limits.”
In a 2020 study, von Biela found that in an average year half of all Yukon kings swimming upriver have heat stress.
And it’s not just the river that’s warming. The ocean is heating up too. Climate change is bringing on more marine heat waves, or periods of severe ocean warming.
Jim Murphy is a NOAA fisheries biologist who has studied salmon at sea for 20 years. He said marine heat waves are disrupting the availability of salmon prey species. It’s not totally clear what’s happening at sea, Murphy said, but when he examines fish, one thing is clear: all salmon — but especially chum — are not getting enough to eat.
“Their stomach contents, the amount of food that they have in their stomach has been declining with warming temperatures,” Murphy said. “They’re likely feeding less in these warm years than in cooler years.”
Scientists say all three of these factors — disease, heat waves, a lack of food — exacerbate each other. A fish that didn’t eat enough is already weaker as it starts its journey up the Yukon. Add a parasite and heat stress, and that fish is a lot less likely to make it to its spawning grounds to reproduce, which means fewer fish next year.
Yukon River fish also have the longest salmon migration paths on Earth, traveling as much as 2,000 miles to get to their spawning grounds.
On top of all this, people along the river have another frustration — commercial fishing. Many residents point to Bering Sea pollock trawlers and a commercial salmon fishery along the Aleutians known as “Area M” that they argue are intercepting salmon at sea that would otherwise be bound for the Yukon.
“It kind of pisses me off a little bit thinking about it. Because it’s the double standard,” said Basil Larson, a subsistence fisherman and resident of Russian Mission, on the lower Yukon. He spoke to a weekly Yukon River Drainage Fisheries Association teleconference for river updates this summer.
Larson said it’s infuriating to see commercial fishermen pulling in hundreds of thousands of chum each season while Alaska Native communities like Russian Mission have gone four summers barely able to fish.
“We’ve been getting restricted and restricted and restricted, and it’s not even funny anymore,” he said.
In recent years, Western Alaska fisheries groups and residents of the Yukon and Kuskokwim Rivers have clamored for tighter regulations at sea, like a cap on chum fishing in Area M and stricter chum bycatch limits in the Bering Sea — but so far, regulators haven’t taken much action.
But Murphy, with NOAA, said even though environmental factors driven by climate change are probably the main culprit for the Yukon collapse, right now, commercial fishing is the one contributor we have control over.
“Most people recognize that [commercial fishing] is not what is causing the collapse of these runs, necessarily. But it is something that can be regulated to mitigate the effects of declining production,” Murphy said.
For now, Yukon River residents are in limbo, waiting to see if fish return. Murphy said it doesn’t look like kings will come back anytime soon. But he said there’s hope for chum.
A 2016-2019 Bering Sea heat wave hit chum salmon particularly hard, but since then, ocean temperatures have subsided and Murphy said, juvenile chum are starting to look healthier.
He said signs are good for a stronger chum run in 2024.