By Ayano Hayes, Watershed Conservation Manager
As spring gives way to summer, we’re excited to share early insights from the 2025 salmon smolt outmigration season in the Lagunitas Creek watershed. Smolts are young salmon that have reached a key life stage after spending a year and a half in the freshwater streams, they are now preparing for the transition to saltwater life in the ocean. This phase, known as smoltification, involves major physiological changes that allow them to survive in a new and very different environment. Their successful migration downstream is critical to their survival and, ultimately, the health of future salmon runs.
SPAWN has a monitoring station in San Geronimo creek, the upper tributary to the mainstem Lagunitas Creek. From April to June, the smolt trap was in place, spanning the width of the creek, capturing everything moving downstream. The trap was checked daily in order to safely and successfully process and collect data on this outmigrating class. Outmigration timing remained consistent, total captures peaking throughout April, coinciding with ideal water temperatures but low flow rates. Here’s what we found…
Highlights from the 2025 Season:
This coho salmon cohort originated from the strong 2023/2024 spawning season. However, as juveniles, they endured very wet and stormy few winters, and benefited from favorable streamflow conditions this spring. The total counts for the San Geronimo creek smolt trap produced 1,210 coho smolts with a population estimate of 3,192 ∓635 individuals. The results are an improvement in recent years for the upper tributaries but less than expected for the entire Lagunitas Creek watershed. Marin Water shared preliminary population estimates for the watershed being around 12,000 coho smolts, which is less than 3 years ago but similar to 6 and 9 years ago. The National Park Service, Point Reyes National Seashore, also shared lower than expected counts out of Olema creek, another major tributary to Lagunitas Creek. Expectations were based on the number of nests laid and the number of juveniles observed before the winter and summer preceding migration for this class. Lower than expected counts may be attributed to low winter survival but a more detailed analysis will be available later this year.
View the graph here showcasing coho salmon smolt population estimates since 2006 for San Geronimo creek: Annual Coho DARR Estimates
The smolt trap season also allows biologists to measure and weigh the smolts since the larger the smolt at the time of outmigration, the higher chance of survival in the estuary and ocean they will have. Based upon the coho smolts captured in the San Geronimo trap, the average length was 99mm with an average weight of 10g, which is smaller in size but significantly lighter in weight compared to previous years data. Coho smolts averaged around 125mm out of Olema. Data on fish weight and size gives a clue to food availability and with smaller fish coming out the the upper tributaries, restoration goals may need to target food and habitat abundance.
Another highlight of the season was that chinook salmon were recorded in the San Geronimo smolt trap. This indicates a second year of successful chinook spawning in the San Geronimo creek watershed. Unlike coho salmon who wait 1.5 years before they begin their downstream migration, chinook juveniles will migrate to the estuary 5-7 months after being born. In 2022, 234 chinook juveniles were captured while this year, following a record year of chinook spawning in the watershed, 747 chinook juveniles were in the San Geronimo trap. They ranged from 40mm to 70mm with several over 80mm. With an increase in chinook salmon presence in the watershed, competition for food may have impacted the coho salmon’s feeding potential.
Threatened steelhead are also found in the San Geronimo smolt trap and this year, steelhead smolt numbers were low but consistent for what is usually captured in the San Geronimo creek smolt trap, around 10 smolts. Steelhead juveniles totaled 595 individuals. The results found from fisheries monitoring within the watershed help inform habitat restoration priorities and give us a real-time look at the health of our salmon populations.
As always, your support helps make this monitoring work possible. Stay tuned for the full season report, and thank you for being part of the effort to protect and restore this critical watershed for the endangered and threatened salmonid species.
