Interagency Ecological Program: Fish catch and associated water quality for the Fall Midwater Trawl Survey, Sacramento-San Joaquin Delta, 1967-2024

The Interagency Ecological Program’s (IEP) Fall Midwater Trawl Survey (FMWT) is a long-term monitoring survey conducted by the California Department of Fish and Wildlife (CDFW) and has sampled annually since its inception in 1967, with the exceptions of 1974 and 1979, when sampling was not conducted. The FMWT was initiated to determine the relative abundance and distribution of age-0 Striped Bass (Morone saxatilis) in the San Francisco Estuary (California, United States), but the data has also been used for other upper estuary pelagic species, including Delta Smelt (Hypomesus transpacificus), Longfin Smelt (Spirinchus thaleichthys), American Shad (Alosa sapidissima), Splittail (Pogonichthys macrolepidotus), and Threadfin Shad (Dorosoma petenense).
The FMWT currently samples 130 stations each month from September to December and a subset of this data is used to calculate an annual abundance index. These 130 stations range from San Pablo Bay upstream to Stockton on the San Joaquin River, and West Sacramento on the Sacramento Deep Water Ship Channel. FMWT sampling data includes 193 unique station locations throughout the San Francisco Estuary; 63 are not currently sampled. Most stations no longer sampled occur in South and Central San Francisco Bay (1968-1979), with others located in San Pablo Bay and upstream into the Delta. There are catch records for stations that were sampled only once. Data users are encouraged to look at station frequency over time to understand when and where sampling occurred. Sampling takes approximately 14 days per month to complete. Historically, FMWT sampling occasionally began as early as July (1972) or August (1968-1973, 1993-1994, 1996-1997) and sometimes continued past December to March (1968-1973, 1978, 1991-2001) or beyond (1992-1995). The Spring Midwater Trawl (SMWT) was a modified survey created to continue sampling January-March to track movements of mature adult delta smelt from 1991-2001. SMWT was replaced in 2002 with the more effective Spring Kodiak Trawl (SKT).
All fish, shrimp, and jellyfish are identified and enumerated. In addition, the crew measures water temperature (°C), specific conductance (µs/cm), Secchi depth, and Turbidity (NTU). FMWT also currently collects meso- (copepods and cladocerans) and macro- (mysids and amphipods) zooplankton samples at 35 stations. Midwater and zooplankton tows conducted by FMWT allow for determination of available zooplankton prey resources.
FMWT equipment and methods have remained consistent since the survey's inception, which allows annual abundance indices to be compared across time. Monthly and annual abundance indices are calculated using catch data from 100 “index” stations grouped into 14 regional “areas”. Monthly indices are calculated by averaging catch per tow for index stations in each regional area, multiplying these means by their respective weighting factors (i.e., a scalar based on water volume) for each area and summing these products for all 14 areas. Annual abundance indices are the sum of the 4 (September-December) monthly indices. The FMWT catch data has been used to meet permit obligations to the State Water Resources Control Board (SW and DRCB) via Water Rights Decisions (D-1485 and D-1641) and USFWS-NMFS biological opinions for Delta Smelt (Hypomesus transpacificus), and for incidental take permits issued by CDFW for operation of Central Valley Project (CVP) and State Water Project (SWP).

For more FMWT information, as well as a user copy of our Microsoft Access, please visit our website:
https://wildlife.ca.gov/Conservation/Delta/Fall-Midwater-Trawl