Recent counts of juvenile coho salmon in Devil’s Gulch and San Geronimo Creeks found essentially no “young of year” coho, fish that would have hatched from eggs laid last winter. The dearth is likely the result of extreme drought, said Marin Municipal Water District fisheries scientist Greg Andrews.
Coho typically lay redds, or nests of eggs, in December and January; eggs hatch in the spring, and the young spend a year in the freshwater creeks before migrating to Tomales Bay and the ocean.
But the drought meant that fish couldn’t access either the small creeks till February, when big rains hit the county. The district counted a few fish, but believe they were probably older juveniles that decided to remain in the creek for an additional year before migrating to the ocean.
According to National Park Service figures, young fish in Olema Creek also suffered, with a roughly two percent survival rate of redds.
The water district believes that by the time adult coho were able to spawn in the two creeks—a month or two after their typical spawning period—the eggs were too old. “We have a suspicion that perhaps, being as late as it was, those females had to hang onto those eggs in her belly, and perhaps they weren’t viable when she did spawn”.
Typically the district estimates Devil’s Gulch and San Geronimo have about 8,000 juveniles in total; this year they might have had less than 150.
Luckily, the main stem of Lagunitas Creek, which receives regulated flows from water district reservoirs, had above average levels of juveniles at last month’s count, with 18,000 counted. (The average is about 15,000.)
“There’s no reason to think the cohort would be wiped out. The fish sampled this fall will come back as adults, and they might to go San Geronimo and Devil’s Gulch. They’re not necessarily so honed in on Lagunitas Creek,,” Mr. Andrews said. “So we may see a decent crop of fish come back and spawn throughout the watershed a year and a half from now.”