Earlier this month, teams of biologists with barrels on their backs hiked three miles of Redwood Creek, scooping out juvenile coho salmon and placing them into pools. The more than 4,000 fish, which came from a hatchery in Sonoma County, are meant to bolster the creek’s small population, which has suffered from inbreeding that’s led to diminished survival in an already treacherous ocean setting. Barely 1 percent of regional coho survive their year-long journey into the Pacific Ocean long enough to return to their natal streams.
The Redwood Creek watershed is one of 25 global biodiversity hotspots recognized by the Nature Conservancy. It spans nearly nine square miles from the peaks of Mount Tamalpais through Muir Woods, where it feeds into Big Lagoon before draining into the ocean at Muir Beach. The lagoon once covered an area of roughly 25 acres, but in 1967, it was partially filled for a parking area. Each year, rains are needed for the mouth of Redwood Creek to open enough for salmon to make it upstream. Enough rain had fallen by mid-December for spawners to begin their upstream migration.
Because the Redwood Creek watershed lies within the Golden Gate National Recreation Area, the National Park Service monitors its health and coho activity. When teams surveyed creeks following last winter’s heavy storms, Redwood was the only coho stream in West Marin that did not have a single coho nest, known as a redd. It was the first time in more than two decades that the creek was empty, but coho had been declining for years. Park fisheries biologist Michael Reichmuth said the cohort leaving the creek in 2020 was already small.
“We weren’t anticipating a high fish return,” he said. “It might have been a combo of the winter storms, but we just didn’t have a lot of fish leave that creek.”
Starting in the early 2000s, coho started to decline precipitously in Redwood Creek. Coupled with a coast-wide coho crash in 2008, prolonged drought and nearly a century of watershed degradation from development, the California Department of Fish and Wildlife foresaw the potential of coho completely disappearing from the creek in 2014. It began introducing hatchery-reared juveniles in a last-ditch effort to stave off extirpation. The initiative lasted until 2019. Yet after placing almost 500 coho into the creek, the project failed to leave a lasting impact.
For its size, the Redwood Creek watershed is disproportionately affected by visitors, according to the park service. In 2019, the park service embarked on a massive restoration of the watershed that included enhancing coho habitat by opening creek beds and tributaries and removing invasive species. The project is expected to be finished by 2029.
From July to November of this year, the park service placed trees and logs, graded banks, planted native species and removed century-old rock walls built to prevent flooding in town. According to Mr. Reichmuth, the watershed was so wood-starved in part because in the 1980s, Fish and Wildlife and the park service removed natural wooded habitat along the creek.
“This project should have been done 20 years ago,” project manager Carolyn Shoulders wrote in a statement. “The changes that we’re making are overdue.”
The thinking at the time was that logs prevented coho from navigating the creek. Mr. Reichmuth said the benefits of the restoration will be realized in the next decade as coho bounce back with the help of hatchery releases. “With the improvements that we’ve done, I think we are giving them a better chance of success, but we definitely needed the support of bringing in additional fish,” he said. “You’re not going to see that great habitat in year one. It takes a while for things to grow back and get the results you hope for.”
This winter’s hatchery release was led by Fish and Wildlife, which directs all other coho recovery projects in the state. It was 20 times larger than the previous hatchery releases at Redwood, and could be followed up by routine releases until the population stabilizes, Mr. Reichmuth said.
The hatchery fish are sourced from Olema Creek and the Russian River watershed, then artificially spawned and reared before their offspring are released at select streams in the Tomales Bay and Russian River watersheds. At Redwood, fish were released into different pools across three miles to avoid overcrowding.
At the hatchery, each fish is implanted with a passive integrated transponder, a chip that contains details such as its length, weight and sexual maturity. A genetic sample is extracted from the fin of each fish and given to the National Oceanic and Atmospheric Administration’s Southwest Fisheries Science Center in Santa Cruz for DNA analysis.
Scientists compile a genetic spawning matrix that details which fish are least like one another, giving biologists a reference for best mating options. Once spawned, the fish eggs are grown in the hatchery until they hatch 40 days later. After the coho fry have developed enough to feed themselves, they are released at different stages of development. Some are nine-month-old juveniles, like the 4,000 placed throughout Redwood Creek; others, known as smolts, are released after a year, when they are ready to leave their natal streams. Other adult coho with their own unique genetic diversities were released for a chance at natural breeding.
This year, 366 adult coho were released in Redwood Creek, Walker Creek and Salmon Creek, north of Bodega Bay. Walker Creek has been receiving juvenile and adult hatchery coho since the early 2000s. Manfred Kittle, a Fish and Wildlife supervisor who helped lead the projects, said that in the last two years, traps have shown that hatchery fish are mating with each other and with wild salmon, boosting population numbers.
Some organizations have been critical of hatchery releases, arguing that they reduce the health of wild salmon. In July, the conservation nonprofit Trout Unlimited published a synthesis of peer-reviewed global research on the effects of hatchery salmonids on wild salmonid populations. According to Helen Nelville, an author of the report, interbreeding between wild and hatchery fish reduces genetic diversity, diminishing the average size and overall abundance of wild fish and compromising their natural reproduction. But she said that in areas where fish populations are nearly extirpated, like Redwood Creek, hatchery releases can play a significant role in breathing life back into habitats.
“When we talk about potential negative effects of hatchery fish, you first need a robust and functioning wild population,” Mr. Kittle said. “But there aren’t any robust populations that could be affected by hatchery fish [in Redwood Creek]. The fish we put out there are most likely the only coho salmon in the creek this year.”
Unlike other Pacific salmonids, coho spend their first year in their natal streams, where they are vulnerable to water quality and habitat degradation. After finding their way to the ocean, they are confronted with predators and pollutants. After a treacherous year at sea, coho return to their natal streams to spawn before dying.
This year’s smolts are showing average performance in West Marin when compared to the robust performance of last year’s smolt cohort and the weaker performance of juveniles in 2021. Redwood Creek has the weakest smolt cohort.
But Mr. Reichmuth remains optimistic. Last week, he observed three redds in Redwood, and said the adult hatchery coho are already spawning with one another. Meanwhile, up north in Bear Valley, his crews counted 150 adults in Olema Creek—the highest single-day tally for adult coho since the park service began counting in 1995.