Runs and Rainfall: When Does Coho Spawning Season Begin?

From the Park Service's San Francisco Bay Area Network salmonid monitoring blog, December 6, 2022, by Watershed Stewards Program corpsmembers Catherine Masatani and Elizabeth Bear, written as the 2022-2023 season began.

With winter coming, the rains that open the spawning season had just arrived. Late-fall storms refill the streams, raising their flow or even reconnecting them with the sea. Without that rain, the coho salmon native to Olema and Redwood Creeks in Marin County could not swim upstream to spawn — the end of a three-year life cycle and the start of the next generation.

Coho salmon returning to Olema Creek to spawn, November 2021. NPS.

Waiting for the first flush

Scientists know the conditions that let salmon migrate, but not the timing, which depends on the weather. The Network's coho and steelhead monitoring crew waits for a first flush, when a watershed's streams are all connected and flowing and fish can move. That usually takes 5 to 7 inches of rain within 30 days.

• Redwood Creek needs flows strong enough to break through the sandbar at its mouth.
• Olema Creek needs only enough water to reconnect dry stretches and raise flows in the shallows.

Spawner surveys can therefore start any time from October to December. Most years the crew surveys often from November to early February, and runs usually end in late January. On average a run lasts about 51 days in Redwood Creek and 46 days in Olema Creek, varying by 18.5 days either way with the rain.

Redwood Creek where it meets the Pacific after a major storm, January 2006. NPS.

What the long record shows

The Network's monitoring links rainfall and streamflow to adult coho migration back to the 1997-1998 season at Olema Creek and the 1998-1999 season at Redwood Creek. It also tracks rainfall for each water year (October 1 to September 30), since more rain often means easier access for spawners.

• Olema Creek had the most rain in the November–February spawning window in water year 1998 and the least in 2021.
• Redwood Creek had the most in 2017 and the least in 2021.
• In 2021 both creeks saw weak spawning, judging by the number of redds (nests) recorded; 1998 and 2017 were strong years.
• On average, Olema Creek gets 26 inches of rain in the spawning period and Redwood Creek 18 inches.

Weather extremes cut both ways. A flood can make it easier for adults to swim in, but can also wash out redds and kill eggs, and the muddy water afterwards makes it hard for surveyors — since coho and steelhead spawning often overlap — to tell fish and redds apart. A dry year makes passage harder and keeps spawners from the best habitat; the water is clearer, but there are few or no fish to see.

As the 2022-2023 season approached, the crew was watching rainfall and streamflow closely, hoping to see coho during December surveys.

More: the Network's salmonid monitoring page, the Pacific Coast Science and Learning Center's coho and steelhead page and its blog.

Sources

• National Park Service, San Francisco Bay Area Network, Runs and Rainfall: When Will the 2022-2023 Spawner Season Begin?, by Catherine Masatani and Elizabeth Bear; rewritten in hubnx's own words.