Low flows equal high stakes for salmon in the Klamath River
Klamath River salmon are facing elevated infection rates of the parasite Ceratonova shasta (C. shasta) this spring, a setback tribal scientists say reflects harsh water conditions rather than any failure of the region's historic dam removal.
Monitoring by the Karuk Tribe has detected higher levels of C. shasta, a long‑standing parasite that spreads more easily when river flows are low and water temperatures rise. This year's record‑low snowpack, early melt and a sudden heat spell created ideal conditions for the disease to proliferate among juvenile salmon.
"It's been a problem for a long time; it's actually native to the Klamath, so it's not a new problem," said Craig Tucker, natural resources policy consultant for the Karuk Tribe. "It's one of those things when the flows are low, and water temperatures are hot, the disease spreads more rapidly, and we have had years where upwards of 90% of juvenile salmon are being infected by it."
The disease C. shasta requires two distinct hosts to complete its life cycle: a salmonid fish (where it proliferates in the intestinal tract) and a tiny, freshwater polychaete worm (Manayunkia occidentalis). The parasite cycles between juvenile salmon and tiny polychaete worms that live in riverbed gravels. Strong winter flows typically scour those gravels and disrupt the worms' habitat. A century of dam operations muted high flows and forced adult salmon to congregate below the structures, increasing disease transmission.
"The dams really dampened winter high flows, so you didn't get the gravel stirred up at the bottom of the rivers," Tucker said. "With dam removal, fish can disperse out through the basin, and these winter flows will move more gravel, so we were a little disappointed to see high rates of infection this year."
This year brought the worst-recorded snowpack and extremely low river flows, creating conditions that could have triggered a severe disease outbreak in Klamath salmon. Tucker noted that a brief heat spell raised risks, but many wild fish migrated out of the basin before spore levels peaked, leaving mostly recently released Fall Creek Hatchery fish affected in the latest infections. He emphasized that while dam removal is already improving natural river function, the Klamath is still in the early stages of recovery, and researchers are closely monitoring the system.
"Polychaete worms live down while they're really small, barely see them with your naked eye, and when they're infected, they release spores that infect fish," said Tucker. "The way you manage those things, is if you get a big flush flow in the wintertime that moves, you know, gravel at the bottom of the river, it sort of disrupts the life cycle of that worm, if you have good flows in the river, it's a little bit of that dilution as the solution to the pollution kind of thing, where you know the more water you have, the more dilute the spore concentration is, and the lower infection rate."
Tucker said dam removal is already helping restore the Klamath River to more natural conditions, but the river is still in the early stages of recovery.
"We've seen immediate benefits; fish have spawned in places they haven't been in a century; this year's low flows created a tough situation," said Tucker. "A lot of fish left the basin uninfected. There's good reason to be hopeful about what comes next."
Tucker mentioned that many wild juvenile salmon migrated out to sea before parasite levels peaked. The majority of recent mortalities appear to be hatchery‑released fish from Fall Creek Hatchery, which may prompt changes in how release timing is coordinated with river conditions.
"Good spring flows for those fish to rise to the ocean are important, but also the bigger and fatter your fish is, the better it survives," said Tucker. "It's not an easy decision. Should we keep it in this hatchery and get fattened up and release it, or if there's good flows in the river that will release them now? It's not always an easy decision. I'm hoping that what we've learned this year and what we've observed this year will help us make better choices next year."
Upper Klamath Lake, the river's headwaters, is managed by the Bureau of Reclamation, which must balance flows for salmon, endangered sucker species and agricultural diversions. In dry years, those competing demands intensify.
"The Klamath River starts as the outflow of Upper Klamath Lake. It's a natural lake, but the Bureau of Reclamation put a dam on the outflow, so they can manage this natural lake almost like a reservoir, and so there are rivers that flow into Upper Klamath Lake, and then the Klamath River flows out of Upper Klamath Lake," said Tucker. "A lot of the battles on the Klamath are how much do you let out of the lake for the river, how much you leave in the lake for suckers, which are an endangered fish, and then how much you take out of the lake for agriculture; that's kind of the water battle on the Klamath."
Tucker said he remains optimistic about the long‑term outlook.
"There are very regular meetings between state and federal agencies and tribes to try to make these choices and decisions, so there's quite a coordination, and you know, dam removal is a work in progress. I think we've seen some immediate benefits to dam removal, but you know it's going to take time for the river to heal and recover from being dammed for a century, and we still have artificial diversions to meet agricultural needs that we have to get right, so that fish will benefit to the maximum extent possible."
In a release, the Karuk Tribe emphasized that this year's disease detections should not be interpreted as evidence that dam removal has failed. Instead, they reflect the complexity of rebuilding a river system that has been altered for more than a century.
"Dam removal gave the Klamath River and our fish a fighting chance," Karuk Vice Chairman Kenneth Brink said in the press release. "But no one should confuse dam removal with an instant cure."
Maranda Vargas can be reached at 707-441-0504
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