Biologists Brave Cold Waters to Count Juvenile Salmon

By Tara Blake and Natale Urquhart

Fisheries Crew Leader Brentley McNeill and Hydrologic Technician Karl Kindall doing a snorkel survey together. Crew members braved the cold and plunged into Redwood Creek's 47°F water to assess winter habitat use by juvenile coho salmon and steelhead trout.

WSP / NPS / Natale Urquhart

February 2022 - With the coho spawner season over, our San Francisco Bay Area Network fisheries crew shifted focus to a different generation of salmonids in Redwood Creek. Deep pools and sheltered banks below the creek’s surface are home to juvenile coho salmon and steelhead trout. Crew members braved the cold and plunged into the 47°F water to assess winter habitat use by these fish.

Fisheries Biologist Michael Reichmuth searches for juvenile fish. Their small size, iridescent silvery color, and tendency to swim near the shelter of large woody debris, boulder structures, and dark undercut banks means they are just about impossible to see from the surface.

WSP / NPS / Tara Blake

Visitors to Muir Woods National Monument may have spotted our crew working up and down the creek donning black wetsuits and snorkel gear. It may seem superfluous for our crew to submerge themselves in the ice cold creek rather than conduct these surveys from the streambanks. “Can’t they see fish from above?”, one may ask. Well, juvenile salmonids are quite small during the winter, measuring around 3-5 inches long. They are an iridescent silvery color, and prefer to swim near the shelter of large woody debris, boulder structures, and dark undercut banks. All that means observation from above the water’s surface is near impossible. Underwater observation allows our crew to get a close-up look at the young salmonids, and a more accurate count.

After two days in the water, the crew counted approximately 20 juvenile coho salmon and steelhead, all of which are at least one year old. These salmonids will migrate downstream in the spring, as they make their way to the ocean as smolts. The crew also spotted around a dozen Chinook salmon fry that recently emerged from their gravel redds (nests). These fry are the progeny of the anomalous Chinook spawning seen this past fall.

Juvenile coho salmon shines in a surveyor’s flashlight on the floor of Redwood Creek. After two daysof snorkeling, the crew counted approximately 20 juvenile coho salmon and steelhead, all of which are at least one year old.

NPS / Michael Reichmuth

Three recently emerged Chinook salmon fry swim along the creek bed. These fish are about the size of a paper clip! Altogether, the fisheries crew spotted around a dozen Chinook fry during their surveys. They are the progeny of the anomalous Chinook spawning seen this past fall.

NPS / Michael Reichmuth

While the winter snorkel surveys did provide a peek at the juvenile numbers in the Muir Woods section of Redwood Creek, that wasn't the main intent. Rather, researchers wanted to see how the young fish are utilizing structures installed during a recent restoration project. In 2019, the National Park Service began the Redwood Renewal Project in Muir Woods. The large-scale, multi-year effort encompasses infrastructure improvements, visitor access, and habitat restoration. One sub-project is the Salmon Habitat Enhancement Project aimed at improving federally endangered coho salmon habitat over two major construction phases. Both phases involve removing riprap (rock walls) from streambanks and introducing large woody debris structures in different creek sections.

Overall, this work will create refuge for emergent fry and rearing habitat for overwintering juveniles. It will also improve egg survival by decreasing potential redd scour during large storms. With Phase 1 completed during the summer/fall of 2019, our focused surveys serve to evaluate the success of those stream enhancements. Our findings will help further improve designs for Phase 2 construction, currently planned for summer/fall of 2023.

The fisheries crew will conduct snorkel surveys again during the summer months. Then, our aim will be to get a better understanding of coho and steelhead spawning success. Hopefully, the water will be a little warmer for the snorkelers among us!

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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