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Snorkeling a cold creek to count young coho salmon in Muir Woods

In February 2022, with the coho spawning season over, the National Park Service's San Francisco Bay Area Network fisheries crew turned to the next generation of fish in Redwood Creek, in the Muir Woods area. Deep pools and sheltered banks hold juvenile coho salmon and steelhead trout, and the crew pulled on black wetsuits and snorkel gear and went into the 47°F water to see how the fish use their winter habitat.

A juvenile coho salmon lit by a flashlight on the creek bed.

A juvenile coho salmon in a surveyor's flashlight beam on the floor of Redwood Creek. NPS photo.

Why get in the water

It may seem easier to count from the bank, but juvenile salmonids in winter are only about 3 to 5 inches long, with an iridescent silvery color, and they favor shelter under large woody debris, boulder structures and dark undercut banks. From above they are close to invisible. Underwater, the crew gets a close look and a more accurate count.

What they found

After two days in the water the crew counted about 20 juvenile coho and steelhead, all at least a year old. They will head downstream in spring toward the ocean as smolts. The crew also saw about a dozen Chinook salmon fry that had recently emerged from their gravel nests, the offspring of an unusual Chinook run the previous fall.

Three tiny Chinook fry swimming just above a gravel creek bed in a flashlight beam.

Three recently emerged Chinook fry, about the size of a paper clip, along the creek bed. NPS photo.

The real aim

Counting the juveniles was not the main purpose. The researchers wanted to see how young fish use structures installed in a recent restoration. In 2019 the Park Service began the Redwood Renewal Project in Muir Woods, a multi-year effort on infrastructure, visitor access and habitat. One part, the Salmon Habitat Enhancement Project, aims to improve habitat for the federally endangered coho in two construction phases. Both remove riprap (rock walls) from streambanks and add large woody debris structures in different creek sections.

The work should create refuge for newly hatched fry and rearing habitat for juveniles over winter, and it should improve egg survival by reducing the chance that storms scour the nests. Phase 1 was finished in summer and fall 2019, so the surveys help judge it, and the findings will help improve the design of Phase 2, planned for summer and fall 2023. The crew planned to snorkel again in summer to learn more about coho and steelhead spawning success.

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