Physical and Biological Drivers of Longfin Smelt Vertical Distribution

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Research scientists study conditions that affect abundance and distributions Longfin Smelt fish population.

Longfin Smelt study sites

Map of the San Francisco Estuary shown with study sites from the CDFW Bay Study. Site 325, highlighted in red, is the location where the longfin smelt vertical distribution studies will be conducted.​​​​​​​

The longfin smelt (Spirinchus thaleichthys) is a small (90-110 mm), schooling, pelagic fish found in estuaries and coastal waters along the Pacific Coast of North America. It exhibits an anadromous life history strategy with rearing in estuarine and marine habitats for 2-3 years before spawning in upstream freshwater portions of estuaries. The southern-most reproducing longfin smelt population occurs in the San Francisco Estuary (SFE) in California and is listed as threatened by the California Department of Fish and Wildlife and was deemed warranted for listing by the U.S. Fish and Wildlife Service. Recent evidence suggests that variability in longfin smelt vertical distribution may affect the interpretation of data on the spatial distribution and abundance of longfin smelt collected in long-term monitoring programs.

The following research questions will be addressed:

  • Are longfin smelt evenly distributed vertically in the water column?
  • Does longfin smelt vertical distribution respond to environmental cues, including gradients in tidal currents, turbidity, salinity and ambient light, diel cycle or food abundance?
  • Do longfin smelt need to achieve a threshold age or size before they respond to environmental cues for vertical distribution?

This 2-year field study will include some initial pilot work and 2 separate studies examining vertical distribution of longfin smelt. The purpose of the pilot effort will be to refine sampling logistics and obtain preliminary information on the efficacy of non-lethal sampling methods for longfin smelt. The vertical distribution studies will be conducted in the San Francisco Estuary within San Pablo Bay. The general design of the vertical distribution studies will be to sample physical and biological parameters approximately every 2 hours for 30 consecutive hours in order to obtain consistent data over a full range of tidal and diurnal cycles. All sampling will be conducted aboard the USGS R/V Turning Tide. The basic approach to the data analysis will consist of two stages: (a) data exploration, which includes generating summary statistics and graphical representations of the data, and (b) model building and the development of statistical tests. The results of the study will be summarized in a peer reviewed journal article.

This study has important management implications to California's water supply. The Central Valley Project and State Water Project provide water from the San Francisco Estuary to approximately 25 million people and a multi-billion dollar agricultural industry in California's Central Valley. Water project operations are, in part, governed by the status and distribution of longfin smelt. This research will generate new information that will contribute to the conservation of longfin smelt and the reliability of California's water supply.