 Delta Smelt(Hypomesus transpacificus)
 Distribution of Delta Smelt in San
Francisco Bay and Estuary
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
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