Banner year for spawning coho

01/23/2019

Salmon spawning season is going strong in West Marin. According to Eric Ettlinger, the Marin Municipal Water District’s aquatic ecologist, this year’s run of coho is the largest in 12 years, with more than 300 redds, or nests, and 700 adults counted so far. 

As of Jan. 11, 60 redds were seen in Devil’s Gulch, 75 in San Geronimo Creek and 39 in the tributaries monitored by the Salmon Protection and Watershed Network. 

The season’s surveys have been complicated by wet weather and partisan politics, however. Heavy rains have made it difficult to navigate muddy creeks and may threaten the durability of nests, and, due to the federal shutdown, National Park Service staff have been unable to conduct spawning surveys for over three weeks. 

Most spawning in Marin is monitored by M.M.W.D. staff, who survey Lagunitas Creek, Devil’s Gulch and Walker Creek. The park service monitors Olema Creek, which accounts for 20 percent of spawning in the Lagunitas Creek watershed. Before the shutdown, the park counted over 50 redds in the creek. Mr. Ettlinger said he hoped to team up with SPAWN to survey the area this week, and Preston Brown, director of SPAWN’s watershed conservation program, said he did not think the lack of park monitoring would be “a big detriment to our knowledge.” 

Coho measure roughly two feet long. They are usually sliver, but males turn red during spawning season, which begins in November. The salmon swim from the ocean into the Tomales Bay, using the rains to migrate into the Lagunitas Creek and its tributaries in order to spawn. The period lasts for three to four months. While the adults die soon after spawning, their offspring live in the creeks for roughly a year and a half before migrating back to sea. 

Droughts are detrimental to the cycle: when water levels dip too low, coho are unable to travel. But heavy rains bring their own concerns—namely that redds will be washed away. Mr. Ettlinger said he didn’t think the past few weeks had brought enough rain to damage redds, but admitted he could not be sure. Because stream gauges operated by the United States Digital Service are not reporting data—a disruption he said was likely linked to the shutdown—he has no data on water levels. 

Furthermore, he said, “any redds obscured by the high flows, and almost certainly redds that were built in the last three weeks but never documented or seen, are going to be impossible to see now because the flows would have flattened the stream bed and obscured those redds.” 

Mr. Brown, too, was optimistic but cautious about the impact of the weather on this year’s run. “If the storms we’ve had are the biggest we’ll see, I think we’ll be okay,” he said. “But if we have bigger ones approaching, the more likely some [nests] will get blown out.” This year’s spawning season is nearly over, he added.

As to the large run this year, Mr. Ettlinger said the cohort saw a high number of smolts go out to sea. “It looks like only about two percent of the fish that went out to the ocean survived and have come back,” he said. But even that low percentage has yielded a high return. 

Yet Mr. Brown cautioned that the numbers are still a far cry from the watershed target of 1,300 nests set by the National Oceanic and Atmospheric Administration. If that number is reached consistently for a decade, the species would be considered recovered.

Mr. Brown attributes much of the coho’s decline to development in the headwaters of the San Geronimo Valley, which he said restricts the creek’s ability to create a truly successful habitat for the fish. 

“One seemingly good year here, one low year there: they even out and it’s still a flat line,” he said. “Until we start to see that each cohort is getting greater and greater, which will take some time, we’re still dealing with a critically endangered species.”