Directed Outflow Project Lower Trophic Study

The upper San Francisco Bay Estuary and Sacramento San Joaquin Delta is critical habitat for the endemic Delta Smelt (Hypomesus transpacificus), an endangered planktivorous fish with an annual, semi-anadromous life cycle. Freshwater outflow actions during the fall season are hypothesized to improve Delta Smelt habitat and therefore Delta Smelt condition before the spawning life stage. As part of efforts to evaluate the effectiveness of such actions to benefit Delta Smelt populations, various studies were initiated by the U.S. Bureau of Reclamation under the Directed Outflow Project (DOP). One component of the DOP was to evaluate how the Fall X2 freshwater outflow action changed the lower trophic prey available to Delta Smelt. The action is hypothesized to change abiotic and biotic aspects that can benefit Delta Smelt habitat by using freshwater outflow to maintain the position of X2 (defined as the distance in kilometers from the Golden Gate Bridge to the tidally averaged 2 ppt salinity isohaline) at around 70 km during wet or above normal precipitation water years. This moves the low salinity zone (0.5 - 6 ppt) further seaward into Suisun Bay which increases the area of preferred Delta Smelt habitat. Field sampling began the September of 2017 and occurred bi-weekly through the end of November, paired with the U.S. Fish and Wildlife Service Enhanced Delta Smelt Monitoring program. Three random sites were sampled per target region (Suisun Bay, Suisun Marsh, the Lower Sacramento River, the Cache Slough Complex, and the Sacramento Deep Water Ship Channel) per week. Sites were chosen using a Generalized Random Tessellation Stratified design. During 2018, sampling was increased to weekly and in 2019-2020 sampling began in April and ended in November. Sampling occurs for three different habitat types: shoal, channel surface and channel deep at each site depending on the depth of the water column. The shoal habitat is less than 10 feet, channel habitat is greater than 10 f