Pathogen-Resistant Salmon Stock May Signal Hope For The Upper Klamath River

A gut-rotting pathogen continues to plague salmon of the Klamath River, but one hatchery broodstock may be best at braving infested waters, according to preliminary results of a research study funded by California Sea Grant. 

The lower four dams of the Klamath are finally coming down (JC Boyle, Copco 1, Copco 2 and Iron Gate Dams), and with them, a new hope for salmon. The river’s native salmon populations have been devastated for decades by poor water quality and the increasingly abundant pathogen, Ceratonova shasta. Introducing hatchery-raised salmon could reinvigorate salmon populations, but only if they can withstand these gut wrenchers. 

California Sea Grant graduate research fellow Leah Mellinger found that broodstock from one hatchery may be better at resisting C.shasta while making the perilous journey to the sea. Her research also suggests that for these small fry, bigger may be better.

Dangerous Declines

The Yurok tribe — whose ancestral home includes the Klamath — has an oral history of the river before the dams, and they want to get it back to what it was, according to Mellinger. 

“This has been a top priority for tribes for a very long time,” Mellinger said.

Historically, the Klamath River has provided bountiful spawning grounds for Chinook and other salmon. For centuries, the Yurok have welcomed back the bronze flashes of spawning Chinook, but in recent decades salmon populations have plummeted. 

Year after year, the tribe has seen “massive fish mortalities,” Mellinger said. “And these mortalities have long-reaching impacts for future generations of fish and people alike.”

In the summer of 2021, more than 70% of the river’s juvenile salmon were found dead and about 80% of juveniles were infected with C.shasta. This stowaway lives a cyclical life—infecting a freshwater polychaete worm which then releases spores to be consumed by fish. Inside the fish, spores mature and are passed back to worms. This cycle of infection can be deadly to salmon, rotting them from the inside out and decreasing their overall health. 

Sink or Smolt

Previous studies showed that Chinook salmon are more resistant to C.shasta infestations than other species of salmon. Still, the impact of C.shasta exposure on juvenile Chinooks’ ability to migrate from spawning grounds and successfully transition into the ocean was unclear. And, if young fish can’t make full transition into the ocean, their introduction could be a waste of resources and another blow to the recovery of the Klamath. 

The most essential step in the migration process is smoltification. Fry must morph their gills to withstand the shift from fresh waters to salty seas, transitioning from freshwater parr to smolt. If they don’t, they quickly die of dehydration. 

Mellinger, guided by the Yurok tribal fisheries department, sought to find the broodstock that could withstand longer exposures to C.shasta and still be able to smolt.

Previous research suggests that Chinook salmon with different ‘run times,’ — meaning the season in which adults return to spawn — vary genetically from each other. Mellinger chose juvenile Chinook broodstocks from two Klamath hatcheries with different run times: Iron Gate fall run and Trinity spring run. 

Salmon Size is Key

Back at Bartholomew Lab at Oregon State University, Corvallis, Mellinger exposed these two broodstocks to C.shasta for a varied amount of time similar to the time the juveniles would naturally spend in the Klamath after hatchery release before testing their ability to successfully undergo a simulated migration to the ocean and subsequent freshwater to oceanic transition. Exposure to fluctuating water temperatures mirrored those along their natural passage, accompanied by a change in salinity signaling their theoretical arrival at the Pacific. Fish were carefully monitored over the following week, during which they’d either sink or smolt. 

C.shasta had devastating impacts on the Iron Gate fall broodstock, killing all of those who were exposed for the longest time with no ability to smolt after exposure. In contrast, the Trinity spring broodstock didn’t seem to be affected by the pathogen at all, and yet, fish were dying. 

Mellinger noticed a curious trend: the spring run broodstock who survived, regardless of pathogen exposure, were bigger. Mellinger extended her study to investigate this clue. She found that spring stock were dying from the stress of the salinity change and their failure to undergo smoltification. As she suspected, the key was size.

Her findings indicated that the Trinity spring run stock is more resistant to C.shasta exposure than Iron Gate fall run, but need to be larger to successfully transition into the ocean.

With further investigation, these findings could lead to clear recommendations for tribal fisheries on the Klamath. If these trends persist, Trinity spring run stock that’s allowed to grow larger than the standard size for release may be best suited to be released in the Klamath after the dams are removed.

Returning to Ancestral Streams

In another study, researchers found that spring run salmon from Trinity hatchery are most closely related to the Chinook salmon native to the Upper Klamath that are now protected under the endangered species act. Trinity hatchery salmon are not only a superior biological fit, but a culturally significant one. 

“Having these fish go back to their ancestral roots and spawning grounds is a spiritual need and a communal need as much as it is scientific and production based,” says Mellinger. 

For Mellinger, the point of her work is to help answer the questions, not ask them. 

“So when I say I didn't come up with this, I really didn't,” Mellinger said. “I came up with a way to try to illuminate a question that the Yuruk tribe had.”

She wanted to do research that would reflect the needs of the Yurok fisheries department and its people; not just for her degree,” she said.

Mellinger has worked alongside Native American tribes her whole academic career. After she graduates, she wants to go back to working with the federal government to help facilitate collaborative partnerships of fishery stocks between stakeholders and tribes.

For the Yurok fisheries department, Mellinger’s findings may help guide the repopulation of the Klamath. Since this study was conducted, Mellinger has been using molecular analyses to further investigate her novel findings. Her current research continues to investigate the relationship between size and smoltification, with the addition of a third run: Central Valley winter run.