Runs and Rainfall: When Will the 2022-2023 Spawner Season Begin?

By Watershed Stewards Program Corpsmembers Catherine Masatani and Elizabeth Bear, San Francisco Bay Area Network Salmonid Monitoring Program

December 6, 2022 - With winter fast approaching, the rain that marks the beginning of spawning season has just arrived. These late-fall rain events replenish our streams, increasing their flow or even reconnecting them with the sea. Without rainfall, the adult coho salmon native to Olema and Redwood Creeks wouldn’t be able to migrate upstream to their spawning grounds. It is truly a special time, as coho salmon are completing their three-year life cycle while attempting to produce the next generation of fish.

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

NPS

Although scientists can study what conditions allow for instream migration of salmon, timing is difficult to predict, since it is highly weather dependent. Therefore, the San Francisco Bay Area Network’s (SFAN) coho and steelhead monitoring crew must wait until there is enough rainfall to cause a first flush—or when the rivers within a watershed are all connected and flowing—allowing for fish movement. Typically, there needs to be 5-7 inches of rain within a 30-day period to initiate such flows. Redwood Creek requires flows strong enough to breach a sandbar at its mouth, while Olema only needs enough water to reconnect dry sections of stream and increase flows in shallow areas. Until this occurs, our team will wait patiently to get back into the creeks for spawner surveys. Since the coho spawning season is so heavily correlated with rainfall, our surveys may begin anytime between October and December. Most years, we conduct frequent surveys from November through early February. The coho runs usually end in late January when spawning fish are finished moving into these coastal Marin watersheds. On average, coho spawner runs last about 51 days in Redwood Creek and 46 days in Olema Creek. Runs can vary in length based on rainfall, deviating from such averages by 18.5 days.

Redwood Creek at the confluence with the Pacific Ocean after a major storm event, January 2006.

NPS

With SFAN’s history of long-term monitoring, we can correlate streamflow and rainfall data with adult coho migration going back to the 1997-1998 spawner season for Olema Creek and the 1998-1999 season for Redwood Creek. In addition to migration timing, we track monthly and annual rainfall averages for each water year (WY), which runs from October 1st to September 30th. This data can reflect the success of a spawner season, as more rain often means easier stream access for spawners. Based on rainfall data collected for Olema Creek, the most rain during the coho spawning window (November through February) occurred in WY1998, while the lowest rainfall amount occurred in WY2021. Meanwhile, Redwood Creek experienced the highest rainfall amount in WY2017 and the lowest in WY2021. Sure enough, in WY2021, there was weak spawning success in Olema and Redwood Creeks, as documented in the number of redds recorded, whereas both WY1998 and WY2017 saw a strong spawning success. Overall, Olema Creek received an annual average of 26 inches of rainfall during the coho spawning period, while Redwood Creek received 18 inches.

This robust long-term monitoring practice has also allowed us to understand how weather anomalies can affect spawner success in more complex manners. For example, a flood event might allow adult coho to migrate into a creek more easily. However, it may also result in egg mortality due to redd scour. High turbidity after storm events also affects visibility for data collection. Since the coho salmon and steelhead spawning windows often overlap, low water clarity makes it difficult for crew members to identify fish and redds to species. A particularly dry year, on the other hand, can make fish passage more difficult and prevent spawners from reaching ideal habitat. Visibility may be improved, but there are few to no fish to be seen.

The monitoring crew has been attentively checking current rainfall and streamflow data in preparation for the onset of the 2022-2023 spawner season, and we’re hoping to see coho in Olema and Redwood Creeks during our December surveys!

For more information

• San Francisco Bay Area Network Salmonid Monitoring webpage
• Pacific Coast Science & Learning Center Coho & Steelhead webpage
  • San Francisco Bay Area Coho and Steelhead Blog
• Contact Fishery Biologist Michael Reichmuth

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