Date

Fall 2025

Document Type

Master's Thesis (Open Access)

Degree Name

Master of Science (M.S.)

Department

Moss Landing Marine Laboratories

Abstract

Estuaries provide many important ecosystem services connecting the land and sea. For example, they act as nurseries by providing food, habitat, and refuge for juvenile fish that support economically valuable fisheries as adults. However, environmental and anthropogenic stressors, such as warming and hypoxia (i.e., low oxygen), have been linked with biodiversity loss, fish kills, and reduced fish growth, marking a major threat to the availability of high quality nurseries. This study evaluated spatial variation in environmental conditions and nursery function of distinct regions in Elkhorn Slough, a large eutrophic estuary on the Central California coast that supports juvenile flatfish populations including English sole, Parophrys vetulus and speckled sanddabs, Citharichthys stigmaeus . I deployed cages holding flatfish at nine sites along Elkhorn Slough’s main channel, monitoring the growth and survival rates of juvenile English sole (August 2021) and speckled sanddab (November 2021) during two three-week time periods. Sensors recorded dissolved oxygen (DO) and water temperature every 20 minutes during each experiment, while salinity and pH were recorded weekly. All sites underwent substantial daily fluctuations in DO and water temperature, with November experiencing cooler, less hypoxic conditions as well as lower site-by-site variability compared to August. In August, English sole caged at upstream, eutrophic sites—characterized by high daytime temperatures, high salinity, and nighttime hypoxia—suffered high mortality rates and low growth rates. English sole caged at one site (Old Salinas River) exhibited high survival and growth rates despite their location in a eutrophic river channel with poor water quality, possibly due to a combination of high prey density, relatively low salinity, and relatively cool temperatures. In November, juvenile speckled sanddabs exhibited higher survival rates than English sole, though survival was still lower at upstream, more eutrophic sites. A non-significant trend toward higher weight loss at seaward sites compared to sites in the Middle Slough may reflect the seasonal reversal of the salinity gradient or other factors not included in this study, such as prey density. My results suggest that water temperature and DO both contribute seasonally to determining the quality of nursery habitat in Elkhorn Slough, highlighting a need for continued monitoring and local management efforts to improve water quality.

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