A serendipitous weather pattern three years ago has resulted in the Lagunitas Creek watershed’s second-most prolific coho salmon spawning season since officials began counting the endangered species in 1982.
So far this season, ecologists have tallied 593 redds, or nests—the most since the winter of 2004 to 2005, when they documented 634.
The salmon that return to the watershed each year were born three years before, and this year’s cohort, it turns out, hatched in an especially propitious year. An early atmospheric river struck in October 2021, and it was followed by two more wet months and then three months of dry weather—a perfect combination for spawning success.
“We had good rain that allowed fish to spawn all over the watershed, up into the little headwater streams, and then we had this very dry period when those eggs incubated undisturbed in the gravel,” said Eric Ettlinger, an aquatic ecologist with Marin Water, which coordinates the counts. “High flows can really wash away or smother a lot of eggs, but we didn’t have that.”
California’s coho salmon have been in steady decline since the mid-20th century due to a variety of factors, including overfishing of ocean stocks and habitat loss resulting from urbanization and other human development. They were once widely distributed and abundant in many of the coastal watersheds of central and northern California, ranging from the Smith River near the Oregon border to the San Lorenzo River in Santa Cruz County.
In Marin County and elsewhere, the ebbs and flows of the population are closely monitored each year.
“Salmon play a number of important ecological roles, including as food for a number of bird species, river otters and scavengers,” Mr. Ettlinger said. “They transport large volumes of nutrients from the ocean, which fertilizes riparian forests and boosts ecosystem productivity.”
But in densely populated areas like Marin, he said, their most important role is as environmental ambassadors.
“People get excited to see big red fish in their neighborhoods and start to see their local creeks in new ways,” Mr. Ettlinger said. “They start to care more about litter and water quality. I’m amazed by how much local kids know about aquatic and marine food webs because they’ve learned about salmon in school. If we were to lose our local salmon, it would be like losing the best environmental educator ever.”
The spawners of the 2021-2022 run didn’t set any records, but their numbers were well above average. Because their eggs did extraordinarily well, the fish that survived produced the much larger run this year. Marin Water is still tallying the final counts but estimates that nearly 1,200 adult coho returned to the watershed this year, a count based on the assumption of two adults per nest.
“The coho run is pretty much over,” Mr. Ettlinger said. “We saw some fish this week, and we expect that to be pretty much the end of the run.”
The counts of coho nests are up in every location where monitoring takes place, including Lagunitas Creek (80), San Geronimo Creek (155), Olema Creek (150), Devil’s Gulch, (126) and the San Geronimo tributaries (82). Overall, this year’s preliminary tally of 593 nests is more than double the annual mean of 247.
Meanwhile, threatened steelhead trout have started to return to the watershed, and ecologists will now turn their attention to counting them.
“Early indications are that this is a much better steelhead run than the previous couple of years,” Mr. Ettlinger said.