| Summary: | The Interagency Ecological Program San Francisco Estuary 20-mm Survey was initiated by the California Department of Fish and Wildlife (CDFW) in 1995 to monitor the distribution and abundance of post-larval and juvenile Delta Smelt (Hypomesus transpacificus). Delta Smelt are considered to be an annual fish endemic to the upper San Francisco Estuary that spawn in early spring and rear through the summer and fall in fresh to brackish water. Young of the Year (YOY) Delta Smelt are considered juveniles at sizes greater than or equal to 20-mm, and this survey targets those Delta Smelt. The 20-mm Survey currently samples 47 stations at fixed locations from San Pablo Bay through Suisun Bay and into the Sacramento-San Joaquin River Delta every other week from March to July. During high outflow years, 5 additional stations are sampled in San Pablo Bay to provide greater spatial coverage of potential Delta Smelt habitat. The survey has historically sampled between 41 and 55 stations. At each station, three 10-minute stepped oblique tows are conducted for fish. Each stepped oblique tow follows a tow schedule to sample the entire water column, and the volume of water sampled is estimated using a General Oceanics flowmeter. The net is a cone shaped plankton net composed of 1600μm mesh and measuring 5.5 meters in length with a mouth area of 1.51m2 lashed onto a rigid D-frame with skis. Post-larval and juvenile fish samples are preserved in the field in 10% Formalin dyed with Rose Bengal and brought back to the CDFW Stockton Lab for identification to species and enumeration under a microscope. At each station, environmental and water quality data are also collected, and include: top temperature, top and bottom electrical conductivity (normalized at 25degreeC), water clarity, and tidal condition.
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