Project Overview
The Summer Townet Survey (STN) is one of the longest running long-term fish monitoring programs. Since 1959, STN has sampled fixed locations from eastern San Pablo Bay to Rio Vista on the Sacramento River, and to Stockton on the San Joaquin River and a single station in the lower Napa River. The survey was initially started to monitor age-0 Striped Bass (Morone saxatilis) and later began to monitor Delta Smelt (Hypomesus transpacificus), a sensitive native species believed responsive to the proper functioning of the Delta region. Data collected at 31 index stations are used to calculate annual relative abundance indices for age-0 Striped Bass and Delta Smelt. The study area was expanded in 2011 to include stations located within the Sacramento Deep Water Ship Chanel and Cache Slough for a total of 40 stations. These stations are sampled every other week June through August.
In addition to fish, the STN also measures environmental variables including water temperature (°C), water clarity (Secchi Depth & NTU) and specific conductivity (µS/cm). Managers and researchers use the data collected by STN to monitor long-term trends in the fish and invertebrate community, monitor the diet and condition of important predator and prey fish species, detect the introduction of new and potentially invasive species, inform decisions and improve the understanding of the health of the San Francisco Estuary.
Additional information on abundance indices, sampling protocol, and copies of the STN databases can be found on the CDFW FTP site for the Fall Midwater Trawl.
History
- 1959 Start of the Summer Townet (STN) Survey to monitor Age-0 Striped Bass at 31 index stations.
- 1990 STN begins to report an annual Age-0 Delta Smelt Index to assess the health of the Delta Smelt population.
- 2003 STN standardizes sampling to 6 surveys annually beginning in June and ending in August.
- 2005 STN adds a zooplankton net and retains a subset of fish to assess food resources and examine fish diets
- 2008 The STN project is mandated by the Delta Smelt Biological opinion for the combined operation of the Central Valley Project and State Water Project
- 2011 STN adds 8 new, non-index, stations to improve understanding of the habitat use of the North Delta.
Methods
Townet Sampling
The townet is a large conical, fixed-frame net which is pulled 2-3 times, obliquely, through the water for 10 minutes at each sampling station. A zooplankton net is fixed to the top of the net and is towed along with the main net. Fish caught within the townet are identified on site and immature or unidentified specimens, along with zooplankton samples, are preserved for identification at the Stockton laboratory.
Age-0 Striped Bass Index
The Age-0 Striped Bass Index is used to help predict future fishery recruitment (i.e. contribution to the adult population). This index is based upon the abundance of age-0 striped bass and is calculated when the age-0 striped bass population reaches an average size of 38.1mm.
Delta Smelt Index
The annual Delta Smelt Index provides a measure of health for the Delta Smelt population. This index, based upon abundance, is calculated as the average of the first and second survey indices.
Relevant Publications
- Summer Townet Bibliography
- Bryant, M. E., and J. D. Arnold. 2007. Diets of age-0 striped bass in the San Francisco Estuary, 1973-2002. California Fish & Game 93(1): 1-22.
- Chadwick, H. K. 1964. Annual abundance of young striped bass, Roccus saxatilis, in the Sacramento-San Joaquin Delta, California. California Fish & Game 50: 69-99.
- Nobriga, M. L., T. R. Sommer, F. Feyrer, and K. Fleming. 2008. Long-term trends in summertime habitat suitability for delta smelt (Hypomesus transpacificus). San Francisco Estuary and Watershed Science 6(1). 13 pages.
- Slater, S. B., and R. D. Baxter. 2014. Diet, prey selection and body condition of age-0 Delta Smelt, Hypomesus transpacificus, in the upper San Francisco Estuary. San Francisco Estuary and Watershed Science 12(3):23.