Officials in California and Oregon have raised concerns following reports of a deadly parasite causing substantial salmon mortality in key Pacific Northwest rivers. The Ceratonova shasta parasite has shown an increased presence in the Klamath River, a vital waterway flowing through Southern Oregon and Northern California, according to the U.S. Fish and Wildlife Service.
The parasite has been linked to the deaths of numerous Chinook salmon, with large groups of dead fish found along the banks of the Scott and Trinity rivers, both tributaries of the Klamath River.
During the 2026 outmigration season, samples collected from March 17 to May 12 revealed a significant parasite load. Of the 696 salmon examined, 46%, or 319 fish, tested positive for Ceratonova shasta. This marks a substantial increase from 2025, when only 22%, or 39 fish, showed evidence of the parasite.
Ceratonova shasta can impact various freshwater species, often causing affected fish to appear darker and exhibit swollen abdomens. The parasite is transmitted to fish through their gills via infectious stages present in the water, which are shed by infected freshwater polychaete worms. Despite the severity of the impact on salmon populations, health officials have confirmed that the parasite poses no threat to human health.
The Klamath River is a significant river system that originates in Oregon's high desert and flows through the Klamath Mountains before reaching the Pacific Ocean in Northern California. The Yurok Tribal Fisheries Department has been actively monitoring fish populations and observed a drought-influenced fish kill among juvenile salmon attributed to a Ceratonova shasta outbreak.