2022 Smolt Trap Recap

By Watershed Stewards Program Corpsmembers Tara Blake and Natale Urquhart, San Francisco Bay Area Network Salmonid Monitoring Program

June 2022 - Each spring, young salmonids begin their migration downstream towards the ocean. From mid-March through late May, the San Francisco Bay Area Inventory and Monitoring Network (SFAN) fisheries crew maintains downstream migrant traps (commonly called smolt traps) on Olema and Redwood Creeks to study them. With these traps, we aim to document the abundance of juvenile coho and steelhead salmon migrating from freshwater creeks to the ocean. Check out this video about the process of installing and running smolt traps, to learn more!

Natale Urquhart measures fish for length and weight on the Olema Creek smolt trap. Equipment pictured includes: buckets for transporting and holding fish (including aerators to oxygenate the water), ruler and scale to measure fish, and instruments used to surgically implant tags into smolts. NPS / Watershed Stewards Program Corpsmember Tara Blake

Smolts are typically 3–5 inches in length and boast shimmering colors along their bodies. Smoltification is a seasonal process occurring in the spring when there is more sunlight on the creeks. It entails physiological changes in the fish. Their bodies become longer and more streamlined, their scales become more silvery, and their gills and kidneys alter their functions to better process salt in the water. These changes prepare them for life in the ocean.

In the 12 weeks that our smolt traps were operating, our crew captured over 1,700 salmonid smolts. Coho salmon made up the majority of our smolt numbers, but Chinook and steelhead salmon were also found on both creeks.

A particular highlight of the season was the capture of over 1,500 coho smolts on Olema Creek. Coho smolts can be distinguished from their Chinook and steelhead counterparts by their larger eyes and hooked anal fin. Based on preliminary estimates, this is the highest smolt outmigration recorded since monitoring began in 2004. It also marks a 10-fold increase in the number of smolts leaving Olema Creek for this cohort. During the winter of 2023-2024 we hope to see roughly 2%—or about 100 to 150—of these fish return as adults to spawn in Olema Creek. Their successful return is dependent upon good ocean conditions and sufficient rainfall.

Coho smolt is netted from the Olema smolt trap. The spots along its body are starting to fade into silver and the tips of its fins are darkening. NPS / Watershed Stewards Program Corpsmember Tara Blake

The second most common salmonid in the traps this spring was the steelhead smolt. We found just under 100 steelhead in our traps. They are generally bigger and spottier than Chinook or coho salmon.

Steelhead salmon smolt on a measuring board, distinguished by its red lateral line and spotted dorsal fin. NPS / Watershed Stewards Program Corpsmember Tara Blake

Our less frequently observed and most anomalous salmonid is the Chinook smolt. We documented about 40 for the first time ever in the smolt traps. They are the offspring of the adults that came up during last fall’s heavy October storms. Where coho and steelhead salmon will stay in the freshwater for about a year before moving to the ocean, Chinook outmigration occurs during the same year they hatched.

A Chinook salmon smolt found in the Olema Creek smolt trap. It is displayed on the measuring board, entirely silver in color and 81mm long in fork length. NPS / Watershed Stewards Program Corpsmember Natale Urquhart

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

See more from the Bay Area Nature & Science Blog

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