West Marin’s creeks and tributaries yielded a healthy cohort of coho smolts this year, with a record-breaking 50,000 leaving the Lagunitas Creek watershed this spring. After spending their first year in their natal streams, the smolts’ real test begins as they try to survive in the open ocean, said Eric Ettlinger, an aquatic ecologist for Marin Water.
“It’s hard to know exactly what the factors are that lead to high survival [in creeks],” Mr. Ettlinger said. “I would like to give some credit to all the restoration work being done, but I think the main factor was that in 2021-22, we had a very dry second half of the winter, so I don’t think any [nests] were destroyed that year. All those surviving juvenile fish need a place to survive; there’s been a lot of work to create shelter for them.”
Meanwhile, the number of chinook and steelhead smolts leaving for the ocean took a dive, though biologists remain unconcerned. Their habitat needs are much less specific than those of coho, which need deeper and more undisturbed waters, and their numbers fluctuate more regularly in West Marin due to strays from hatcheries in the Central Valley.
Lagunitas Creek, West Marin’s most populous tributary, had 26,000 coho smolts, a number extrapolated from a raw count conducted by biologists with Marin Water and the National Park Service over nine weeks starting in early April. Traps placed at the bottom of the watershed are used to count and mark coho, which mostly migrate at night.
Olema Creek, a major tributary of Lagunitas Creek, yielded 24,000 smolts, a massive jump from last year, when there were just 1,500.
“Olema Creek has never had anything close to what it had this year,” Mr. Ettlinger said. “Part of it seems to be that some of the fish that were juveniles in Lagunitas Creek went up into Olema [Creek] during the winter. Like those in Lagunitas Creek, they also had great incubation and juvenile numbers.”
Coho stay in their natal streams for a year before they go to the ocean, unlike most salmon, which leave after a few months. This year’s smolts hatched in early 2022, and only a tiny fraction of them will survive ocean conditions to return in the winter of 2024. Smaller male coho known as “jacks” will return to try their luck in spawning in just six months.
As the smolts make their way through Tomales Bay and into the ocean, they will face predation by cormorants, seagulls, terns and more. Once at sea, they will feed on plankton and other invertebrates until they are big enough to eat small fish like anchovies or rockfish, all while avoiding being eaten by seals, otters and other fish.
Mr. Ettlinger said West Marin could see a huge return in 2024 if the smolts survive at an average rate, but he is hesitant to make any predictions.
“We could see 1,000 coho back in the creek, which could mean 500 [nests],” he said. “But I really don’t want to get my hopes up. There’s a lot of warm water in the ocean, which is not productive for plankton.”
The Salmon Protection and Watershed Network monitors coho in San Geronimo Creek, a tributary of Lagunitas Creek. According to Ayano Hanes, a watershed biologist for the nonprofit, smolt numbers were lower than expected, given the number of nests counted in the winter of 2021.
“I think that the fish were flushed down into Lagunitas Creek during the heavy storms,” Ms. Hanes said.
Last winter’s storms came late in the season, and their impact on salmon varied according to life cycle stage. Early rains are the best because they allow adults to travel into different areas with ease, while creating diverse habitat and flushing sand and silt out of the creek. But late storms can destroy nests, so biologists say next year’s smolt populations will likely be smaller.
“With these big storms, it’s always a balancing act,” Mr. Ettlinger said. “There are lots of small streams in the watershed and if we don’t have early rains, the adults are confined to Lagunitas Creek, which always is going to have a good flow, thanks to the water district.”
Next summer, Marin Water will begin its $10 million restoration project in the watershed to further enhance year-round habitat for coho, steelhead trout and freshwater shrimp. Peters Dam at Kent Lake prevents needed woody debris and gravel from moving downstream in Lagunitas Creek and its tributaries, so the district plans to meticulously place 300 logs and gravel beds at 11 sites to deepen pools.
The project is expected to continue until 2030.