A parasite that thrives in warm conditions adds to Yukon River salmon woes

Alaska scientists are teasing out the effects of Ichthyophonus, a parasite that has taken a particularly severe toll on already weak Yukon River Chinook runs.

When Stan Zuray began noticing white spots, a mushy texture and a weird “sour watermelon” smell in the Chinook salmon he was pulling from the Yukon River, he knew something was going wrong.

That was back in the 1980s. By about 1990, he and others just stopped fishing what he calls “the ass end of the king run” because the fish were so low in quality, he said.

Everyone stopped fishing earlier than normal, he said. “You just get tired of the bum-looking fish,” said Zuray, who lives in the village of Tanana but still has his Boston accent after spending half a century in the Alaska Bush.

It was an ominous turn for a river famous for producing some of the world’s biggest and most prized wild salmon. Chinook salmon, also known as king salmon, is so exalted among the five Pacific salmon species that it is designated Alaska’s official state fish. And the Yukon River Chinooks were traditionally considered among the best of the best.

The culprit in the mushy, weird-smelling salmon was Ichthyophonus, a parasite found in oceans around the world, but a particular menace to the health of Yukon River Chinook salmon.

Zuray is known nationally as a featured character in the reality TV show “Yukon Men” and from his own YouTube broadcasts and a biographical documentary. Within Alaska fishing circles, he is well known as a former subsistence representative on the Alaska Board of Fisheries.

In the science community, he is known for his work as one of the early Yukon River Ichthyophonus researchers. He set up a program and documented infection rates at Rampart Falls, a spot on the river near the tiny Alaska village of Rampart, collecting data that has become critical to current studies.

Zuray said he could smell the infection as fish dried on racks in the traditional way and the parasite multiplied rapidly in the warm sunlight. He dissected samples and documented how Icthyophonus attacks a salmon’s heart. “I squish it in my hand by closing my fingers, and the heart will ooze out through my fingers,” he said.

Waves of infection as salmon runs dwindle

Since Zuray began documenting Icthyophonus in the late 1980s, there have been waves of infections — periods when the parasite was so prevalent that half or more of tested fish were infected, followed by slower periods, followed by new increases.

Those waves, especially the most recent infectious wave around 2022, coincided with a devastating crash in Yukon River Chinook runs. They also coincide with a long-term warming of the Chinooks’ freshwater habitat

Once thought of as a fungus but now classified as a protist, a category that lies somewhere between fungi and bacteria, Icthyophonus was first identified more than a century ago. It exists in oceans all around the world, as well as in some freshwater bodies. Fish typically become infected by consuming it through prey, but can also get infected through physical contact or absorption of water.

As with other parasites affecting fish, the warming climate is a factor in the spread of Ichthyophonus. It multiplies faster in warmer conditions, and fish enduring warmer conditions are more stressed and weaker, making them more vulnerable to infection.

Alaska fish known to get Ichthyophonus infections include herring, halibut and pollock, for which the parasite can be a minor nuisance.

But for Yukon River Chinook, once plentiful and the backbone of commerce, communities and culture, Ichthyophonus is adding to the long-term problems plaguing the runs. It can kill fish, and those that survive infections can be in weakened states, with meat that is not dangerous but unpalatable.

Yukon River Chinook numbers have plummeted. In 2022, the Chinook return to the Yukon River was about 80% below the previous 30-year mean, according to federal scientists.

The picture at the far end of the river’s spawning grounds is especially grim. In recent years, less than a third of the minimum number of adult Chinook salmon needed to sustain the population have made it back to Canadian spawning grounds. Chinook salmon fishing has been closed entirely for several years on the Canadian portion of the river, and closures will likely continue until 2030, under an Alaska-Canada agreement signed in 2024.

Exactly what role Ichthyophonus plays in the decline is yet to be determined, but it could be important, said Alaska Department of Fish and Game pathologist Jayde Ferguson.

“From a research priority perspective, it ranks high to figure it out,” said Ferguson, who works at the department’s fish pathology lab in Anchorage. The lab tracks numerous fish diseases, but Ichthyophonus has become a major focus.

The department and various partners, including the U.S, Fish and Wildlife Service, Alaska Pacific University and tribal entities, are spending a lot of time and money trying to find answers “not because we think we can reverse the infection but to better manage the runs as they are now,” Ferguson said.

Northern Journal contributor Max Graham can be reached at [email protected]. The website is merging with the Anchorage Press; you can read the announcement here. To support Northern Journal with a subscription, click on this link.