May 1, 2025

Creek Notes: So long, smolts

Spring is a season of graduations, when young folks mark the transition from one life phase to another – sometimes boldly, sometimes hesitantly. In Lagunitas Creek, thousands of salmonids are currently graduating from their relatively safe freshwater homes and venturing into the vast Pacific Ocean. 

Each species keeps its own schedule. Chinook salmon spend the winter and spring in the creek and leave when they’re less than four inches long. Coho salmon reside for over a year and leave when they’re four to six inches long. Steelhead generally spend at least two years in the creek, reaching lengths of between five and ten inches. All of them transition from the spotty, camouflaged coloration of stream fish into the silver color of ocean fish. During this transitional phase, they’re called smolts. 

Marin Water has operated a smolt trap near Point Reyes Station since 2006. The trap, called a rotary screw trap, consists of a rotating cone suspended between two pontoons. Smolts are swept into the cone, and then interior baffles move them down into a live box, where the Marin Water fisheries team retrieves them each morning. Most fish are counted and sent on their way, but a subset of fish are marked and released upstream. Recapturing some of these marked fish allows us to estimate how many smolts we’re missing. What do we expect to see from the smolt graduates?

What do we expect to see from the smolt graduates? 

As of this writing, we’re seeing a steady increase in coho smolts, but expect a surge any day now. Over the years we’ve learned that coho prefer to migrate under a new moon but will leave in a hurry during hot weather. Will ongoing cool temperatures mean a big new moon migration, or will we have to wait for warm weather before we see peak graduation? We’ll go to the trap each day, open the box and find out.

New Fish Monitoring Report

Want to dive into more details of Marin Water’s fish monitoring work? Check out the 2023-2024 Juvenile Salmonid and Smolt Monitoring Report.