The rite of passage for young coho salmon is a glorious rendezvous with the sea, but three years of drought have left many migrating fish marooned in the drying tributaries of Marin County’s San Geronimo Valley, according to a recent study.
Many of these trapped juvenile fish, commonly known as smolts, have either been plucked out of isolated pools by birds and other predators or died from lack of nutrients, biologists with the Salmon Protection and Watershed Network said.
“When they are not going out to sea, they are stranded in the watershed, and they are probably dying either from predators or from poor water conditions,” said Christopher Pincetich, a watershed biologist for SPAWN and co-author of the Coho and Steelhead Smolt Outmigration study. “We commonly see the great herons and blue herons fishing in these isolated pools.”
The full extent of the death toll is not known, but the study of coho migration in the Lagunitas Creek watershed during this spring and summer bolstered previous evidence that smolts are at least being delayed before they reach the ocean. The survey found fish in the creek that were at least six months past the age when they would normally enter the ocean and begin feeding on krill.
“A 2-year-old coho should be out in the ocean, but we saw a bunch of them that were just swimming out this year,” Pincetich said. “They were probably trapped last summer, which was a record dry year in Marin County.”
The stranding of smolt trying to reach the ocean is one of a litany of problems facing the endangered Central California coho population, which registered the lowest number of egg-laying adults in the normally bountiful Lagunitas watershed in recorded history last winter.
Big wild run
The 102-square-mile watershed, which winds 33 miles through Samuel P. Taylor State Park and the picturesque oak-studded valley on the northwest side of Mount Tamalpais, supports the largest wild run of salmon along the central coast and is considered a model for fish restoration around the state.
It is unique in that the primary spawning grounds are in the middle of developed communities. Some 40 percent of the coho in the watershed are hatched in tributaries surrounded by homes, golf courses, roads and horse corrals in the 9-square-mile San Geronimo Valley, according to the study.
It is estimated that between 3,000 and 6,000 coho swim down the waterway back to the ocean every year. During this year’s migration, between March and June, there appeared to be a higher percentage of larger juveniles, Pincetich said.
Researchers collected some of the bigger fish, counted their growth rings and discovered that 81 percent of them were at least 2 years old. Coho normally swim back to the ocean when they are 18 months old.
It was the first time since the annual survey began four years ago that 2-year-old coho were found in the Lagunitas watershed. Pincetich said coho have been known to sometimes stay in rivers longer than 18 months, but he believes the older fish in this study were among the hundreds of coho smolt that were trapped last year in drying ponds.
None of that bodes well for the future. Last winter, only 26 coho egg nests, or redds, were counted in the entire watershed, the lowest number in the 15 years that records have been kept by SPAWN and the Marin Municipal Water District. Those fish will be heading back to the ocean next spring. If they run into the same dry conditions and get stranded, they could be wiped out, Pincetich said.
The plummeting coho numbers exacerbate a near-catastrophic decline in the overall population of salmon along the West Coast. The coho population around the state has declined precipitously over the years and so few chinook salmon returned to spawn in the Sacramento-San Joaquin River system the past two years that ocean fishing had to be banned in California and Oregon.
Prime habitat
Besides being prime salmon habitat, the water from the Lagunitas system is the primary source of drinking water for Marin County. Seven dams have been built since 1873, blocking 50 percent of the historic salmon spawning habitat.
The need for more water and the desire to protect salmon has been a delicate balancing act for Marin County, which is building a desalination plant to bolster a deficient water supply and recently completed a Salmon Habitat Enhancement Plan focused primarily on Lagunitas Creek.
