Authorities in California and Oregon are on high alert after recent reports of deadly parasites in popular rivers that are killing salmon.
The Klamath River, a river that flows into southern Oregon and northern California, is experiencing an increase in the parasite Ceratonova shasta, according to the U.S. Fish and Wildlife Service.
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As a result, large concentrations of Chinook salmon were found along the banks of the Scott and Trinity Rivers, tributaries of the Klamath River.
WEITCHPECK, CALIFORNIA – June 9, 2021: Two dead juvenile salmon lie in a bucket after being removed from a Yurok Tribal Fisheries Department spinner trap on the Klamath River in Weitchpek, California. The Yurok Tribal Fisheries Department is monitoring a mass mortality of juvenile salmon due to drought conditions caused by the C. shasta outbreak along the Klamath River. Due to the severe drought, water levels in the Klamath River have dropped significantly since the beginning of the year, resulting in slower flows and warmer water temperatures—an environment in which Cyclone Shasta thrives. Yurok representatives expect Cyclone Shasta to wipe out nearly all of the juvenile Chinook salmon in the Klamath River, negatively impacting not only the fish farming industry but also the Yurok Tribe, California's largest federally recognized tribe, whose culture, ceremonies, and traditions revolve around the annual fish spawning. (Getty Images)
According to the National Wild and Scenic Rivers System, the Klamath River is one of three rivers that traverse the Cascade Mountain Range. It originates in the interior high desert of Oregon, then cuts through the Klamath Mountains and empties into the Pacific Ocean in Northern California.
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During the 2026 migration season, from March 17 to May 12, law enforcement officers collected large numbers of salmon to measure parasite loads in the fish.
As a result, it was found that 46% (319) of the 696 fish collected were found to have the deadly parasite.
WEITCHPECK, CALIFORNIA – June 9, 2021: Gilbert Myers, a fisheries technician with the Yurok Tribal Fisheries Department, removes fishing traps from the Klamath River in Weitchpek, California. The Yurok Tribal Fisheries Department is monitoring a drought-related mortality of juvenile salmon caused by a Ceratonova shasta (C. shasta) outbreak in the Klamath River. Due to the severe drought, water levels in the Klamath River have dropped significantly since the beginning of the year, resulting in slower flows and warmer water temperatures—an environment in which C. shasta thrives. Yurok representatives expect C. shasta to decimate nearly all of the juvenile Chinook salmon in the Klamath River, negatively impacting not only the fishery but also the Yurok Tribe, California's largest federally recognized tribe, whose culture, ceremonies, and traditions revolve around the annual fish spawning runs.
This is a significant jump from 2025, when only 22% (39) salmon showed these traits.
This parasite can affect several species of freshwater fish and typically causes the fish to darken in color and become enlarged.
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WEITCHPECK, CALIFORNIA – June 9, 2021: Jamie Holt, lead fisheries technician for the Yurok Tribal Fisheries Department, attempts to contain the spread of river lamprey by sorting fish caught in a screw trap on the Klamath River in Weitchpek, California. The Yurok Tribal Fisheries Department is monitoring a mass mortality of juvenile salmon due to drought conditions caused by an outbreak of Ceratonova shasta (C. shasta) along the Klamath River. Due to the severe drought, water levels in the Klamath River have dropped significantly since the beginning of the year, resulting in slower flows and warmer water temperatures—an environment in which Cyclone Shasta thrives. Yurok Tribe members expect Cyclone Shasta to wipe out nearly all of the juvenile Chinook salmon in the Klamath River, negatively impacting not only the fish farming industry but also the Yurok Tribe, California's largest federally recognized tribe, whose culture, ceremonies, and traditions revolve around the annual fish spawning run.
According to the Alaska Department of Fish and Game, this parasite is transmitted to fish through the gills by infective actinospore stages of Tetractinomyxon, which are present in water and excreted by infected freshwater polychaete worms.
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Despite the alarming news, the Ceratonova shasta parasite does not pose any danger to human health.
Original article: A deadly parasite is killing hundreds of salmon in a well-known river in the Pacific Northwest.
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