Since 1997, Marin Water’s fisheries crew has been documenting the annual return of adult salmon and steelhead to the Lagunitas Creek watershed. Surveys are focused on counting coho salmon redds – the nests they build out of streambed gravel – and these counts are our most reliable way to estimate the numbers of coho returning each year. Our annual estimates of adult returns provide the best measure of the health of the critically endangered coho population.

Why don’t we count the fish directly?

In some places, such as the Russian River, biologists take advantage of river-spanning structures to count fish as they migrate upstream. Advances in camera technology and AI are even automating that work in some places. On Lagunitas Creek, muddy water and debris make such systems nearly impossible. Our crews must therefore go out every week between November and March and find new redds.

How do we know if a coho salmon built a particular redd?

Sometimes the answer is easy. While walking the creek, we often see female coho actively building or defending their redds, allowing us to confidently identify them. Other times, however, we encounter unoccupied redds and must conduct a little CSI – a Coho Spawning Investigation.

We look for multiple lines of evidence:

• Which species have been observed nearby?
• How large is the redd? Steelhead redds, for example, are typically smaller than coho redds.
• What kind of substrate was moved? Coho often leave sand in their redds, while Chinook tend to move larger rocks.

Using these clues, we classify many unoccupied redds, though some remain cold cases (unclassified).

This year, classification has been especially challenging due to a record number of Chinook salmon, the presence of a few chum and pink salmon (two rare species), and unusually early steelhead – all spawning during the same period as coho. To date, we have counted 347 redds, but 91 have not yet been classified. Once spawner surveys wrap up in March, we will review all available evidence and attempt to identify the fish behind each mystery redd.

More caveats than coho?

Through the end of January, Marin Water surveyors had counted 77 coho salmon redds, which – taken at face value – would make the current run one of the smallest in the last 30 years. However, we counted more adult coho than we did three years ago, which is a hopeful sign of generational improvement. The final coho redd estimate is also likely to show generational improvement once unoccupied redds are classified. 

This is an ongoing investigation, and more information will be shared as we learn more – stay tuned.