Delta Smelt (Hypomesus transpacificus)

Delta Smelt, Peter Johnson USFWS
Delta Smelt, Peter Johnson USFWS ( More species photos)

Most Recent

April 18, 2017
Wanted: More smelt data
Enhanced Delta Smelt Monitoring Program helps fill the void

Delta smelt data, Steve Martarano USFWS

Technician Brynn Pearles records data from
daily tows on the Delta in March 2017.
Credit: Steve Martarano, USFWS

Three years ago, the Service, with the cooperation of other state and federal water agencies, began an early warning monitoring program which provided the Service, and state and federal water managers valuable near real-time information about the location of Delta smelt.

Visit Lodi Juvenile Fish Monitoring Program page

June 23, 2016
Mathematician Uses Big Data to Save Tiny Smelt

For more than 30 years, Ken Newman has used his statistical methods to help endangered fish and wildlife species in some of the most complex ecosystems across the U.S. and Scotland. Yet, his work with the Delta smelt may be his most daunting task.

March 29, 2016 UC Davis Coastal and Marine Sciences Institute Delta and Longfin Smelt Symposium Delta and Longfin Smelt: Is Extinction Inevitable?
This video is an overview of smelt issues: The Ecology and Management of Delta Smelt: New insights into the most important non-game fish in the United States

January 2015 An updated conceptual model of Delta Smelt biology: our evolving understanding of an estuarine fish. Interagency Ecological Program of the San Francisco Bay/Delta Estuary, Management, Analysis and Synthesis Team (MAST).


There are four significant threats to the species:

  • direct entrainments by State and Federal water export facilities;
  • summer and fall increases in salinity,
  • summer and fall increases in water clarity,
  • and effects from introduced species.

Ammonia in the form of ammonium may also constitute a significant threat to the delta smelt. Additional potential threats include predation, entrainment into power plants, contaminants and small population size.

Existing regulatory mechanisms have not proven adequate to halt the decline of delta smelt since the time of listing as a threatened species.

We are unable to determine with certainty which threats or combinations of threats are directly responsible for the decrease in delta smelt abundance.  However, the apparent low abundance of delta smelt in concert with ongoing threats throughout its range indicates that the delta smelt is now in danger of extinction throughout its range.

Water Projects Biological Opinions ("OCAP")

On December 15, 2008, the Sacramento FWO issued a biological opinion (BO) on the Long-Term Operational Criteria and Plan (OCAP) for coordination of the Central Valley Project and State Water Project. The Service determined that the continued operation of these two water projects, as described in the plan, was likely to jeopardize the continued existence of the delta smelt and adversely modify its critical habitat. See the Water Projects Biological Opinions ("OCAP") page for more information.

Smelt Working Group

Delta smelt, USFWS
Delta smelt by metric ruler
Photo: USFWS

The Smelt Working Group consists of experts in delta smelt biology from the U.S. Fish and Wildlife Service, U.S. Bureau of Reclamation, U.S. Environmental Protection Agency, California Department of Water Resources and California Dept. of Fish and Wildlife. See bottom of the Water Projects Biological Opinions ("OCAP") page for more information.