Delta Smelt in San Francisco Bay and Estuary

Delta Smelt(Hypomesus transpacificus)

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Distribution of Delta Smelt in San Francisco Bay and Estuary

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The delta smelt is a small, slender-bodied fish with a typical adult size of 2-3 inches that is found only in the Sacramento-San Joaquin Estuary. Historically, it was one of the most common species in the Estuary, however, the population declined dramatically in the early 1980's. The delta smelt was listed as a threatened species by the U.S. Fish and Wildlife Service in March 1993 and by the California Fish and Game Commission in December 1993. Delta smelt are considered environmentally sensitive because they only live one year, have a limited diet, and reside primarily in the interface between salt and freshwater. In 1993 abundance increased in an apparent response to an increase in available habitat brought about by a wet winter and spring which ended the seven year drought.

The delta smelt population is affected by the amount of outflow from the Estuary which varies from year to year due to precipitation and water management. A positive significant relationship between the fall midwater trawl abundance index and the number of days the entrapment zone (where salt and fresh water meet) is in Suisun Bay from February through June has been observed. This suggests that the delta smelt population does better when outflow is allowed to flow downstream and create nursery habitat for delta smelt in Suisun Bay.

Delta smelt and the related species of smelt, wakasagi, are not closely related. Electrophoretic analysis indicated that they are not genetically similar making hybridization unlikely.

Annual abundance indices for the Delta smelt are generated using data from the Midwater Trawl Survey, Midsummer Townet Survey, and the