Juvenile coho salmon have been spotted in Pine Gulch Creek for the first time in more than a decade. A team of monitors conducting a snorkel survey of the Bolinas stream found about 80 specimens of the endangered fish last month, the National Park Service said.

For Michael Reichmuth, a fisheries biologist with the park service, it’s gratifying to see the salmon return to Pine Gulch. The salmon population in the creek crashed soon after he started his job in 2004, and the species was extirpated from the creek by 2010. “It’s nice to see them come back,” he said. 

“In the salmon world, there’s lots of things to get you down,” Mr. Reichmuth added. “When you get these slivers of light, it helps the morale a little bit.”

The National Park Service began monitoring Pine Gulch, the primary freshwater input to Bolinas Lagoon, in the early 2000s, when a small but reliable population of coho salmon would return every year. But when Mr. Reichmuth spotted a carcass that resembled a coho in the creek last winter, he was taken aback, having not seen one there for more than a decade. 

The salmonid monitoring crew also found a redd, or fish nest, that suggested spawning salmon. But their suspicions weren’t confirmed until this July, when crews surveying the creek for fish habitat potential found dozens of juvenile salmon. 

Like other salmon, coho migrate from the ocean to the upper reaches of rivers and streams to spawn in the winter. The eggs hatch in late winter or early spring and the juvenile fish spend the first one to two years of their lives growing larger in the freshwater stream where they are born. Coho then spend one to three years out in the ocean before returning to the same stream to spawn, restarting the cycle. For a population of coho salmon to remain viable, the flow of water in its stream must be reliable and relatively high. 

The Marin Resource Conservation District spent nearly two decades on a project to increase stream flows in Pine Gulch Creek to make it more friendly for coho salmon, using funding from the California Department of Fish and Wildlife to enlist three Bolinas farmers for help. Peter Martinelli’s Fresh Run Farm, Dennis and Sandy Dierks’s Paradise Valley Farm and Warren Weber’s Star Route Farms all agreed to relinquish their water rights to the creek, which they had used to water their row crops. In exchange, the farmers built four storage ponds that draw water during the winter and release it back into the creek in the summer to improve flows. The ponds were completed in 2015.

Elise Suronen, the conservation program manager at the Marin R.C.D. who worked on the ponds project, was excited to hear that the salmon had returned. “Because of this project, there’s more water in the creek during the sensitive summer months,” she said. “I’m hopeful that the project will support them coming back.”

Mr. Reichmuth said the reappearance can’t be attributed directly to the R.C.D. work. The adults that spawned in Pine Gulch were likely strays that traveled up from the mouth of nearby Redwood Creek because that creek was so dry last year, he said. But like Ms. Suronen, he is confident that the project will allow the fragile Pine Gulch population to blossom. He said the work ensured that creek flows are strong, even in July during a severe drought. 

“If it was this same drought before the ponds, I would be a little concerned about those fish surviving,” he said. With a healthy creek fed by new ponds, he said, the juvenile coho will have a fighting chance of growing large and making it back out to the ocean.