Watershed sees record migration of young coho

06/26/2014

About 20,000 young coho salmon—the largest recorded number and almost double the previous high—made the journey from the Lagunitas Creek watershed to Tomales Bay this spring, according to the Marin Municipal Water District, which has been tracking the population for nine years.

If food in the ocean is plentiful, the district says 10 percent could survive to adulthood and return to the creek to spawn, and those 2,000 coho would comprise the biggest return in half a century.

It’s tentative good news for the silvery fish, whose numbers dropped 70 percent in California between the 1960s and 2002, when it was designated an endangered species north of San Francisco. Recent years saw migrations as low as 2,000 or so, in 2010; the last largest smolt migration the water district tracked was 11,000, in 2012. 

The district’s fishery program says unusual conditions this year—strangely enough, the devastating drought—may be responsible for the jump, and that the smolts’ smaller size could also make them more vulnerable to predation once they reach the ocean. The agency is applying for grants to create more habitat so the watershed can support a larger population even in more typical years.

The coho’s life begins in the winter, when adults lay nests of eggs, or redds, in fresh water, a process known as spawning. The eggs hatch into tiny fish called fry in the spring. They spend a year growing in the creeks, and in the next spring migrate to bays and the ocean. Most spend about two years in saltwater before returning to creeks to spawn and begin the cycle anew. (The adults themselves give up their lives for the next generation, dying after laying their eggs.)

Every fall, Marin Municipal Water District and groups like the Salmon Protection and Watershed Network and the National Park Service count the number of fry in the watershed. But no matter how many fry they count, they typically never see more than 6,000 or 7,000 smolts—more mature juveniles—leaving Lagunitas and Olema Creeks for Tomales Bay. In 2008, for instance, they counted over 36,000 fry; in 2009, only about 11,000. But both years saw similar rates of smolts leaving the watershed.

Eric Ettlinger, an aquatic ecologist with the water district, said the smolts typically crowd in the lower reaches of the creek, where physical constraints cap their number, and wait for the right moment to swim into the salty waters of Tomales Bay. 

But this year, he said, without the flows from winter rains, the young fish remained spread out and stranded around the watershed, preventing them from congesting the lower reaches. They waited along the main stem of Lagunitas Creek, as well as San Geronimo, Olema and Devil’s Gulch Creeks, until the February rains allowed them to start their migration. 

During the drought, Devil’s Gulch, for instance, became completely disconnected from Lagunitas Creek. In that small tributary, two-thirds of the fry survived. Mr. Ettlinger called it a “phenomenal survival rate.”

“The hypothesis we’re working on is that, for some reason, the lower creek can [typically] hold around 6,000 smolts… Juvenile coho are really territorial, and at a certain density, they won’t allow new fish to join them. The smaller fish or the later arrivals get pushed downstream, out of the creek, before they are physically ready for saltwater, or they get to the ocean and there’s nothing for them to eat yet,” he said.

It’s possible that gentler creek flows could also decrease mortality, but Mr. Ettlinger said that past years with few storms didn’t produce such striking numbers, and no recent small-scale restoration would account for the jump, either. The best hypothesis is that the dispersal of the fish allowed a drastically large number to survive.

Michael Reichmuth, a fishery biologist at the Point Reyes National Seashore, said he tags smolts and tracks when they pass by an antenna near the lower reaches of Olema Creek. The park’s data supports the water district’s hypothesis, he said.

The banner year for smolts was especially welcome since it comes on the heels of a disappointing spawning season for Marin’s adult coho.

The district originally had big hopes for the season: the adults who returned to spawn in the watershed last winter were from the 2012 cohort of 11,000 smolts—the previous record—and the ocean, where they matured into adulthood, had an abundance of nutritious plankton. But the drought acted to delay and undermine this winter’s spawning. 

By the end of the season, the adults laid 203 redds, well below the 19-year average of 246. 

“They went when ocean conditions were really good, so we hoped that the record number of smolts going to an ocean with a lot of food would result in a lot of adults coming back. But what we saw was far fewer than that,” Mr. Ettlinger said.

He was disappointed, but many watersheds in California had no spawning at all this winter; by comparison, Marin was lucky.

Whether this year’s record smolt migration will bring big numbers of adults to spawn during the 2015-2016 winter season depends on ocean conditions over the next two years, and on creek conditions when they return.

And this particular bunch of smolts might struggle while at sea, since the 20,000 fish were smaller than normal, averaging about 101 millimeters compared to a typical 115 millimeters. “That doesn’t sound terribly different,” Mr. Ettlinger said, “but the smaller the fish, the more mouths they can fit into. There are a lot of hungry fish in the ocean, and [the smolts] need to grow really quickly so they’re too big for some of those predators. I am a little cautious to say that this is just nothing but good news, because they have a lot of challenges ahead.”

It’s possible that some of the smolts will hang out near the Giacomini Wetlands or spend time growing in Tomales Bay before heading out into the treacherous ocean, but the district doesn’t know exactly how long they spend in the bay before heading further afield. 

An El Niño event, which has a 70 percent chance of setting in sometime this summer and 80 percent chance by fall and winter, could also compromise their survival rate by affecting the availability of food in the ocean. But according to NOAA’s National Weather Service, forecasts of its severity have changed in the past month or so and it is now expected to be a moderate event. Louis Botsford, a professor at the University of California, Davis, who studies marine fishery populations, said if the El Niño is indeed weak, “I don’t think their survival would be any worse than in a typical year.”

Even if the El Niño is stronger than expected, retired salmon biologist Paul Siri, who served on the water district’s technical advisory committee for almost two decades, said the large number offers hope of a healthy return. “Twenty thousand increases the likelihood of survivorship at each phase. It’s a very important foundation for this cohort, in terms of their return in a couple years,” he said.

For its part, the water district is working to create more habitat, and is applying for funding from the California Department of Fish and Wildlife to install structures of woody debris in the reaches of Lagunitas Creek, near the intersection of Sir Francis Drake Boulevard and Platform Bridge Road. It’s there the coho could find additional refuge, and possibly keep larger numbers alive—though the project won’t get off the ground till 2016 at the earliest, not quite in time for these smolts, who will begin to return in late 2015.