Monitoring Crew Finds Critically Low Numbers of Juvenile Coho in Redwood Creek

The coho and steelhead monitoring crew completed juvenile coho salmon monitoring in Redwood Creek in August. Here, crew members measure the length and weight of juvenile salmonids captured via seining. They also take small tissue samples from juvenile coho for genetic analysis. NPS

August 2020 - This month, the coho and steelhead monitoring crew completed juvenile coho salmon monitoring in Redwood Creek. They started monitoring on Olema Creek as well, but but had to put that work on hold for several weeks due to the Woodward Fire.

Normally, juvenile coho monitoring would entail snorkel surveys, plus electrofishing. However due to the coronavirus, this has not been a normal monitoring year. Since electrofishing requires crewmembers to work in close proximity, the crew did multiple snorkel survey passes instead. As an alternate way of capturing fish, they conducted seining. The crew also took the opportunity to test a new snorkeling protocol that they have been working on over the last two years. It will yield a second estimate of juvenile population size that they can evaluate when they analyze their data this fall.

Crews have been able to resume snorkel surveys in Olema Creek, as long as air quality remains at a level that is safe for outdoor activity. NPS / Michael Reichmuth

Early survey results show a decrease in this coho cohort’s juvenile population to a critically low level. The monitoring crew counted only 51 coho while snorkeling over 7.5 km of the Redwood Creek mainstem. They counted the majority of these fish within a 1 km reach in Muir Woods. With seining, they captured a subset of those fish in order to gather measurements like length and weight (proxies for fish health). They also take small fin clips for genetic analysis. The team’s preliminary calculation for average coho length is 71 mm, which is similar to the historic average. Both the critically low numbers and the distribution of juvenile coho suggest that only one coho redd successfully produced fry this spring. The monitoring crew was not entirely surprised. Over the winter they counted only three live adult coho on Redwood Creek. They plan to keep a very close eye on this cohort of Redwood Creek coho to help determine if further management action is needed to ensure its persistence within the watershed.

For more information

• San Francisco Bay Area Network Salmonid Monitoring webpage
• Pacific Coast Science & Learning Center Coho & Steelhead webpage
• Contact Fishery Biologist Michael Reichmuth

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