RUSSIAN RIVER, CA, 5/26/23 — At the turn of the century, the plight of coho salmon on the Russian River was severe — so severe that the Russian River Coho Salmon Captive Broodstock Program was initiated in 2001 to prevent extirpation (or localized extinction) of coho in the river. Scientists at the Broodstock Program at Don Clausen Fish Hatchery in Sonoma County have worked to pull the fish back from the brink in the decades since, with the eventual goal of re-establishing self-sustaining salmon runs in the watershed.
A new study published in Conservation Letters offers genetic rescue — a captive breeding intervention that crosses an at-risk species’ population with the same species from another geographic area — as a viable method to keep Russian River coho salmon from disappearing.
In the 2000s, coho numbers in the Russian River had dwindled to the extent that inbreeding among a very small number of salmonids had grown troublesome. Scientists working in the captive breeding program could only create so much genetic variation, even when selecting for as low relatedness as possible between breeding pairs.
So, researchers decided to cross Russian River salmon with coho from Olema Creek, in nearby Marin County. Scientists began crossing the fish populations at a rate of around 25% per year in 2008, and monitoring adult returns — which proved promising. But the method has historically been controversial. While the fish are of the same species, salmon generally return to the same watershed throughout their lifecycle, meaning that localized genetic adaptations are likely.
“Introducing novel genetic variation can have the opposite effect that you intend,” explained Kasey Pregler, the paper’s lead author and a postdoc at the University of California, Berkeley’s department of environmental science, policy, and management. “But recent publications that have come out with meta analyses over these outcrossing studies have shown that with these really small inbred populations, there are consistent benefits to increasing gene flow with nearby populations. It’s the devil we know versus the devil we don’t with inbreeding versus outbreeding depression.”
In this case, the devil we don’t know seems not to have been so bad after all. Scientists did not find evidence of outbreeding depression — when crosses between individuals from different populations have lower fitness levels or maladaptive traits. For fish released in Mill Creek, a tributary of the Russian River, researchers found that hybrid salmon’s survival probability was higher than that of nonhybrid salmon.
However, researchers also found that the magnitude of survivability varied by stream location and time of release (fall versus winter). Fall-released fish tended to have a higher probability of survival; fall-released salmon also spend between four and six fewer months in the watershed.
Mariska Obedzinski, an extension specialist with California Sea Grant who helms salmon monitoring efforts in the Russian River, was not surprised by this finding.
“One of the things that we’ve learned from monitoring the whole watershed is just how much the salmon really need the whole watershed,” she said. “It’s not like they just stay in their stream their whole life and then leave. There is a lot of diversity in how they use different habitats at different times in their lives. Restoration actions that help increase the amount of diverse habitats in the whole watershed, and the connectivity between those habitats, are really important.”
Obedzinski’s team monitors salmon using Passive Integrated Transponders (PIT) tags, each with a unique number that allows scientists to track the different genetic cross types, essentially by waving a wand over the water as they walk along streambeds. Researchers also place antennas along the bottom of the stream that act as “a FastTrak for fish” to track survival.
In one case, they realized fish were not surviving on Mill Creek because of an old flashboard dam that, during most flows, would entirely block salmon passage from critical perennial habitat to a spawning area. Partnering with other agencies, resource conservation districts, and consultants, they were able to remove the barrier.
Genetic rescue has produced promising results for Russian River salmon fitness. But environmental changes could turn the tide for salmon.
“The long-term goal of this whole conservation hatchery program is that the coho can be self-sustaining, that there’s enough habitat to support them to be able to complete their life cycle on their own and we don’t need a hatchery anymore,” Obedzinski said. “Genetic rescue is a way to buy time while we’re restoring the habitat and the flow.”
Pregler is glad that genetic rescue seems to have been a good fit for this watershed. She also sees it as an endorsement of a multi-pronged approach to restoration work.
“I do hope this adds to the evidence that this can be a really valuable option within our conservation toolbox to help recover the species,” she said. “I think another important outcome of this paper is that we saw the variation in genetic rescue outcomes across time and space, potentially owing to environmental conditions. I hope that also helps guide conservation and management. We need to look not just within the genetics work, but also make sure we’re making those habitats more resilient as well, [so that] those two things are happening in tandem.”
In coming years, the Nature Conservancy and Mendocino County Resource Conservation District are pursuing habitat restoration and floodplain connectivity efforts to aid salmon — as well as downstream populations — in the nearby Garcia, Navarro, and Ten Mile watersheds.
Read the full manuscript and findings from the Russian River study online. Beyond the contributions of Pregler and Obedzinski, Stephanie M. Carlson and John Carlos Garza helped frame the paper and provided feedback on multiple manuscript iterations, Garza and Elizabeth A. Gilbert-Horvath provided genotype data and guided the crossing design, and Benjamin White provided early life-history data.
Note: Kate Fishman covers the environment & natural resources for The Mendocino Voice in partnership with a Report For America. Her position is funded by the Community Foundation of Mendocino, Report for America, & our readers. You can support Fishman’s work with a tax-deductible donation here or by emailing [email protected]. Contact her at KFishman@mendovoice.com or at (707) 234-7735. The Voice maintains editorial control and independence.