Deadly parasite is flourishing in a California river

Wildlife experts in both California and Oregon report they’re seeing a high number of newly released Chinook salmon sickened and killed by a parasite.

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The salmon have been found dead at multiple traps in Oregon and California in the Klamath River. The deaths, first reported by Oregon Public Broadcasting, are believed to primarily be due to a parasite called Ceratonova shasta. The parasite, which is linked to the salmon, has reached farther north in the river than ever before following the destruction of four dams near the Oregon-California border. 

C. shasta is a microscopic myxosporean  parasite that looks like a tiny clear kidney bean. It only exists in the Pacific Northwest and can be deadly to Chinook salmon. The parasite in spore form can latch onto the gills of the fish, infecting the animal and causing hemorrhaging. Although the parasite can grow in the fish, it then finishes its lifecycle via tiny freshwater worms, which release the spores that cause the infection. The parasite is not harmful to humans.

Sascha Hallett, an assistant professor at Oregon State University, told SFGATE that “dead and dying juvenile Chinook salmon” were visible in both traps and along the banks of the river in recent weeks.

A from the U.S. Fish and Wildlife Service found that almost half, or 46%, of nearly 700 salmon found in traps have tested positive forC. shasta. The parasite is naturally occurring.

“What we get concerned about is when those infections develop into severe infections and disease, and when that disease culminates in premature death of these fish,” Hallett said. 

She said the cases they’re seeing now are different from in the past, when severely sick fish often died before the parasite could mature. This meant the parasite would be removed from the population and not cause more disease. But in this case, they’re seeing sick and dead fish containing mature parasites that can potentially lead to new infections.

“It’s unusual that there’s such a severe reaction and the parasite has still been able to mature,” she said. “So it’s double whammy.”

One problematic factor for the fish actually occurred months ago, when Northern California and Oregon saw record low snowpacks. In the northern Sierra Nevada, the snowpack was just 6% of average on April 1. The paltry or nonexistent snowpack meant there was little snowmelt in the river in the spring, which means the river is running warmer and at lower levels than in previous years.

“If the water becomes too warm, they become stressed, and they’re not as capable of dealing with infections,” Hallett said of the salmon. The warm water temperatures also led to the parasite proliferating quickly. 

“When the [pathogen] dose that these fish receive is high, that’s when we see these infections culminate in disease and death,” Hallett explained.

The large number of sick and dead fish comes weeks after the California Department of Fish and Wildlife released 675,000 juvenile salmon in the river in mid-May. The fish were screened for disease and released right before storms, which would cool the water and raise its level, according to CDFW spokesperson Peter Tira.

Stephen Atkinson, an associate professor of microbiology at Oregon State University, told SFGATE that the newly released juvenile salmon remained in the same area for a few days before moving out to the ocean. 

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“It’s just unfortunate that they were hanging around in warmer water that had parasite spawns in it,” he said.

Atkinson said in addition to C. shasta, researchers have also seen another parasite called Parvicapsula minibicornis sickening the fish in the river.

The Klamath River has been drastically changed in recent years after four large dams were removed, allowing the Chinook and other species to swim up the river to Oregon for the first time in a century. This spring was the first time the Chinook were seen spawning in Oregon since before the final dam was removed in 2024.

The return of Chinook to the area is extremely important for local tribes, including the Klamath, who have been pushing for the removal of the dams for decades, and who also monitor the river for water quality. The Treaty of 1864 granted the Klamath, Modoc and Yahooskin-Paiute water rights along with hunting, fishing and gathering in the Klamath Basin. But the federal government effectively ignored the treaty (like hundreds of others), leaving the tribes to fight to remove the dams, push to improve water quality and have fish return to their ancestral waterways.

Hallett said the waterway is in “transition” after the dams were removed. She explained that experts understood the parasite would appear farther north along with the fish, but they didn’t anticipate that “the parasite would be so abundant so quickly.” 

Currently, many of the juvenile salmon are swimming out to the ocean, where the surviving fish will grow dramatically before they return in a few years and swim upriver. The adult salmon generally return in the late summer and fall to travel hundreds of miles upstream to spawn.

Miles Daniels, an associate researcher at UC Santa Cruz, said there’s a risk the adult fish could die before reaching spawning grounds as they’re exposed to more pathogens on their route.

“The longer you’re in a room with someone that’s sick, the more likely you’re going to get infected,” he explained. “The longer the fish is in that river, in that hot spot where there’s maybe a high density of C. shasta, the more likely they’re going to probably get infected.”

Hallett said she and other experts are watching closely to see what happens in the next few weeks and whether the parasites remain the major cause of death for fish or there are other sources they haven’t identified.

“We don’t have the complete picture yet,” she said.