Dismally low numbers of adult coho salmon are returning to the Lagunitas Creek watershed this winter, despite adequate streamflows. Out of the nearly 8,000 smolts that migrated to sea in 2018, the Marin Municipal Water District estimates just one percent have returned to spawn.
“In early December, things were still shaping up as I expected,” said Eric Ettlinger, an aquatic ecologist with the water district. “We were expecting a below-average run and the numbers looked a little below average but nothing alarming. And then the numbers started to decline, when in most years they would be ramping up. In a number of weeks, I went from a little concerned to very concerned.”
The total number recorded across Lagunitas, San Geronimo and Devil’s Gulch Creeks on Jan. 8 was 123 coho and 41 nests, known as redds, which is just a third of the district’s documented average for the time of year. There is still a chance for that number to improve slightly, as spawning peaks between December and the end of January, but it’s a record-low year for the past decade.
There are several possible explanations for the troubles of this cohort, born three years ago. Michael Reichmuth, a biologist at the Point Reyes National Seashore who monitors coho in Olema Creek, said drought conditions in 2017 and a heavy storm in the spring of 2018, right before the fish were leaving the watershed on their journey in the ocean, may have weakened the fish.
Ocean conditions are another factor worrying Mr. Ettlinger. “For a little fish, the key is to hit the ocean and grow fast before getting eaten, which requires getting to the ocean when plankton production is peaking. Was it a poor year for plankton, or were our fish somehow out of sync with the plankton bloom?” he asked in a recent press release.
Dr. William Sydeman, the president and senior scientist of the Farallon Institute, said krill populations were indeed low in 2018 when these fish would have been relying on them for sustenance. The institute, which has tracked krill since 2000, doesn’t yet have the data for last year.
What impact does a bad year have on the species? Mr. Ettlinger said it may not be cause for too much concern; the species is still reproducing in the creek and next year’s spawning generation could have more success. Last year was a banner year, with the largest run in 12 years.
Preston Brown, the conservation director for the Salmon Protection and Watershed Network, said it was important to look at overall trends.
“Every year, we go through the same motion: we look at the past few years. How did this year compare with last year? This year kind of took a lot of people by surprise because these numbers are so low, but we need to look at all the data. The overall trend is that populations are increasingly declining. We may have bumps along the way, but let’s keep this broad picture in mind: the trend is downward.”
Coho numbers dropped by 70 percent in California between the 1960s and 2002, when the species was designated endangered north of San Francisco. The federal coho recovery plan states that the population must maintain a spawning population of 2,600 fish over a 10-year period to be moved from its endangered status to threatened.
Mr. Brown said that over the past two decades of monitoring in the Lagunitas watershed, there have been an average of 500 adult fish and 250 redds per season—a far cry from the target numbers.
The watershed, which hosts the southernmost population of coho in the state, is considered vital to the restoration of the species. Accordingly, SPAWN, the National Park Service, the water district and other agencies have undertaken numerous projects over the past two decades.
Most recently, SPAWN completed a $3 million restoration in the ghost towns of Tocaloma and Jewell, included lifting 13,000 cubic yards of fill out of the creek to create floodplain. The group removed retaining walls, created side channels and installed woody debris.
The group is now in the final stages of permitting a pair of restoration projects on the former San Geronimo Golf Course property that will repair a fish ladder and make improvements upstream on San Geronimo Creek. Mr. Brown said he hopes the group will begin the projects by summer.