Trends in Fish and Invertebrate Populations of Suisun Marsh January 2018 - December 2018 Annual Report for the California Department of Water Resources Sacramento, California Teejay A. O'Rear*, Peter B. Moyle, and John R. Durand Department of Fish, Wildlife, and Conservation Biology Center for Watershed Sciences University of California, Davis October 2020 *Corresponding author:
[email protected] SUMMARY Suisun Marsh, at the geographic center of the northern San Francisco Estuary, is important habitat for native and non-native fishes. The University of California, Davis, Suisun Marsh Fish Study, in partnership with the California Department of Water Resources (DWR), has systematically monitored the marsh's fish populations since January 1980. The study’s main purpose has been to determine environmental and anthropogenic factors affecting fish distribution and abundance. Abiotic conditions in Suisun Marsh during calendar-year 2018 returned to fairly typical levels following the very wet year of 2017. Delta outflow was generally low, with higher-than- average outflows only occurring in April when Yolo Bypass flooded. Salinities in 2018 were about average, in part because of Suisun Marsh Salinity Control Gates operations in late summer. Water temperatures were mild, being higher than average in winter and autumn and slightly below average during summer. Water transparencies were typical in winter and spring but, as has become a pattern since the early 2000s, were higher than average in summer and autumn. Dissolved-oxygen concentrations were consistent throughout the year, with only two instances of low values being recorded, both in dead-end sloughs. Fish and invertebrate catches in Suisun Marsh in 2018 told two main stories: (1) many fishes benefit from higher flows and lower salinities in Suisun Marsh while some invasive invertebrates do not; and (2) Suisun Marsh is disproportionately valuable to fishes of conservation importance.