Earlier this month, in an act of urgency after finding many young Chinook salmon dead in the vegetation along the Upper Klamath River near the Oregon-California border, scientists drove to an Oregon expert with several of the fish carcasses they normally would have mailed.
Dead fish typically sink to the depths of the river or become food for scavengers, so the number of carcasses raised alarms.
“These are juveniles, they’re small, so for there to be so many that you can see groups of them along the sides of the river and in the vegetation along the sides of the river, that’s concerning,” said the expert, Oregon State University fish parasitologist Sascha Hallett, who has studied parasitic species in the Klamath Basin for more than two decades.
Even before Hallett dissected the fish she could tell they had died from a lethal waterborne parasite the size of a blood cell: ceratonova shasta.
For the last few months, state, federal and tribal scientists have become increasingly concerned about the fast spread of the parasite in Chinook salmon, who are carrying it into the Upper Klamath River and Chinook spawning grounds in Oregon for the first time.
Key threat
Ceratonova shasta is found only in the Northwest and it only attacks salmon and trout. It has been around for at least a century, causing large-scale fish die offs in the Lower Klamath Basin in California during the past two decades, and is considered a key threat to the survival of several endangered salmon and steelhead runs in the river.
But four former hydroelectric dams that were removed in 2024 and that had inhibited fish migration into the upper river, also curbed the parasite’s movement, Hallett explained.
Scientists hoped the removal of those dams would improve river conditions and temperatures, and prevent mass disease, infection and die-offs commonly caused by warm stagnant water. But ongoing drought and abnormally warm air and water temperatures are creating the perfect conditions for the parasite to thrive, Hallett said.
U.S. Fish and Wildlife Service officials at the California-Nevada Fish Health Center reported June 1 that 46% of the nearly 700 young Chinook salmon they tested in three locations on the Upper Klamath river near the Shasta River between mid-March and mid-May were infected with the parasite.
About one-fifth of those infected carried lethal levels of the parasite, which is incubated in a worm about the size of an eyelash and spread through the water. Once the parasite enters the fish through its gills, it moves to the intestines and begins to spread, causing hemorrhaging.
On a call Thursday morning with dozens of scientists, federal fish and wildlife biologists told Hallett and others that at one testing area near the Fall Creek Hatchery near the site of the former Iron Gate Dam that used to block the Upper and Lower Klamath, more than 50% of the Chinook found in traps were dead, and at another site 80% were dead, she said.
First time in Oregon
Even before she got the samples driven to her lab from California, Hallett several weeks ago got some young Chinook carcasses from the Oregon Department of Fish and Wildlife that were found in part of the river west of Klamath Falls, and that showed signs of advanced disease from the parasite.
“What’s significant is this is the first time we’ve had mortalities of Chinook juveniles in Oregon since dam removals,” she said.
Scientists assumed the parasite would move further upriver as the salmon returned to former spawning grounds in Oregon that were previously out of reach, but they did not expect so much of the parasite would appear in the Upper Klamath so quickly, she added.
She and a team of researchers at Oregon State, along with federal, tribal and California partners, will continue monitoring the relationship between river temperature and the proliferation of the parasite and studying dead Chinook in the Upper River to learn more about how quickly and severely it’s moving through their bodies.
She said the dead Fall Creek Hatchery Chinook oddly did not immediately begin migrating toward the ocean this spring once released, but hung around the area where the parasite was prevalent for several weeks.
“Some of the questions are: why didn’t they move downstream?” Hallett said. “Further downstream the parasite is not present at such high levels, so if the fish could have moved downstream sooner, they wouldn’t receive such a high dose, and they would have gotten to the ocean sooner too, where it’s cooler water, which would have helped slow down parasite development.”
She described the river as a “system in transition” since the dams were removed, but the discovery of the parasite’s expansion as “shocking and unfortunate.”
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