Long-term monitoring of coho salmon and steelhead during freshwater life stages in coastal Marin County: 2017 annual report

Reichmuth, Michael; McNeill, Brentley; Carlisle, Sarah

The San Francisco Bay Area Network (SFAN) of the National Park Service (NPS) Inventory and Monitoring Program monitors populations of coho salmon (Oncorhynchus kisutch) and steelhead (O. mykiss) in watersheds at Point Reyes National Seashore, Golden Gate National Recreation Area, and Muir Woods National Monument. The watersheds, including Olema Creek, Redwood Creek, and Pine Gulch Creek, represent the majority of streams supporting salmonids in Marin County, California, and support the southernmost stable populations of coho salmon on the Pacific Coast. The NPS SFAN Inventory and Monitoring Program and California Department of Fish and Wildlife have supported monitoring of multiple coho salmon life stages in coastal Marin County watersheds since 1998. Monitoring includes: • adult escapement surveys, which document the number of adult salmonids that successfully “escape” ocean fisheries and return to their natal streams to spawn, as well as the number of redds (gravel nests where salmonids lay their eggs) created; and • outmigrant smolt trapping, where the number of smolts (at least year-old juvenile salmonids that have undergone changes to cope with a marine environment) are captured, measured and counted as they make their way towards the ocean; and • basinwide juvenile coho surveys and summer index reach monitoring which seek to quantify the number of juvenile salmonids present in the watersheds during the summer months. Multiple life-stage monitoring allows for a more accurate representation of the coho population, especially given the complex life history of the coho salmon. Coho salmon have a three-year life-cycle in which they spend the first year of life in their natal stream followed by nearly two years in the ocean before returning to their natal stream to spawn. Since generations of related fish reproduce every third year, the coho population is divided into three distinct year classes, or cohorts. Cohorts are labeled 1, 2, and 3. Cohort 1 was the first to be monitored by NPS in 1997-1998 as one generation returned to spawn in the winter and the next generation hatched and emerged in spring and were surveyed as juveniles during the summer. These fish were surveyed again when they left the creeks as smolts in 1999 and for a third time as spawning adults in the winter of 2000-2001. Succeeding generations of cohort 1 hatched in 2001, 2004, 2007, 2010, 2013, and 2016. Cohort 2 are those fish that hatched in 1999, 2002, 2005, 2008, 2011, 2014, and 2017 and cohort 3 fish hatched in 2000, 2003, 2006, 2009, 2012 and 2015. This report presents a summary of monitoring results for the 2016-2017 spawner season, 2017 smolt trapping season, and 2017 summer juvenile surveys. The adults and juveniles monitored belong to different generations of cohort 2, while the year-old outmigrating smolts belong to cohort 1. Adult coho escapement surveys on Olema Creek for 2016-2017 showed a decrease in cohort 2 adults returning to spawn (15 redds in 2016-2017 compared to 32 redds in 2013-2014), while Redwood Creek showed an increase in returning adults (16 redds in 2016-2017 compared to 5 redds in 2013-2014). Pine Gulch Creek had no adult coho observed for the seventh year in a row. A basinwide visualization technique was used to calculate the abundance of juvenile coho residing in Olema and Redwood Creeks during the summer of 2017. On Olema Creek, a basinwide coho population estimate of 400, 95% CI [181:619] was calculated. The juvenile coho population estimate for Redwood Creek was 1,106, 95% CI [785:1,427]. The Olema Creek population estimate was lower than the last time this cohort was observed as juveniles. In contrast, Redwood Creek juvenile coho population estimate was higher than the previous time this cohort was observed. No juvenile coho were observed on Pine Gulch Creek. The Olema Creek coho smolt production estimate of 5,677, 95% CI [4,538:6,816] was slightly lower than expected given the range of the summer 2016 juvenile....

Long-term monitoring of coho salmon and steelhead during freshwater life stages in coastal Marin County: 2017 annual report

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