The delta smelt (Hypomesus transpacificus) is an endangered slender-bodied smelt, about 5 to 7 cm (2.0 to 2.8 in) long, in the family Osmeridae. Endemic to the upper Sacramento-San Joaquin Estuary of California, it mainly inhabits the freshwater-saltwater mixing zone of the estuary, except during its spawning season, when it migrates upstream to fresh water following winter "first flush" flow events (around March to May). It functions as an indicator species for the overall health of the Delta's ecosystem.
Because of its one-year lifecycle and relatively low fecundity, it is very susceptible to changes in the environmental conditions of its native habitat. It is listed as a federally threatened species under the Endangered Species Act of 1973. Efforts to protect the fish from further decline and extinction have focused on limiting or modifying the large-scale pumping activities of state and federal water projects at the southern end of the estuary, thereby limiting water available to farming. However, these efforts have not prevented the species from becoming functionally extinct in the wild.
Ov
OviparousOviparous animals are female animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive...
Mi
MigratingAnimal migration is the relatively long-distance movement of individual animals, usually on a seasonal basis. It is the most common form of migrati...
D
starts withThe delta smelt is endemic to the Sacramento–San Joaquin River Delta in California, where it is distributed from the Suisun Bay upstream through the delta in Contra Costa, Sacramento, San Joaquin, and Solano Counties. The delta smelt is a pelagic (lives in the open water column away from the bottom) and euryhaline species (tolerant of a wide salinity range). It has been collected from estuarine waters with salinities up to 14 parts per thousand.
Historically, delta smelt were distributed from San Pablo Bay upstream to Sacramento on the Sacramento River and Mossdale on the San Joaquin River, which varied seasonally and with freshwater outflow. Today, large areas of historic delta smelt habitat and designated critical habitat have become unsuitable for some life history stages of the species, though key environmental characteristics (e.g. temperature, salinity, water depth) of these areas have not changed. Delta smelt disappeared from the southern portion of their historic habitat in the late 1970s, which coincides with substantial increases in the amounts of water exported from the delta. Water export operations likely have a great effect on the distribution, abundance, and genetic diversity of delta smelt.
The delta smelt is semelparous, living one year and dying after its first spawning. Their spawning occurs in spring in river channels and tidally influenced backwater sloughs upstream of the mixing zone where salt water meets fresh water. The Sacramento and San Joaquin Rivers then transport the delta smelt larvae downstream to the mixing zone, normally located in the Suisun Bay. Young delta smelt then feed and grow in the mixing zone before starting their upstream spawning migration in late fall or early winter.
The delta smelt is preyed upon by larger fish, especially striped bass and largemouth bass, which are introduced species in the Sacramento-San Joaquin Delta.
Historically, delta smelt were relatively abundant in the upper Sacramento-San Joaquin Estuary, with populations declining dramatically in the 1980s. They were listed as threatened by both federal and state governments in 1993, and sustained record-low abundance indices, prompted their listing as endangered under the California Endangered Species Act in 2010. Critical habitat was listed for delta smelt on December 19, 1994.
Delta smelt are threatened with extinction due to anthropogenic alterations to their ecosystem, including urbanization, non-native species, water diversions, contaminants, and the conversion of complex tidal habitats to leveed channels. A survey in April 2015 found only one individual delta smelt. Although the fish is almost extinct in the wild, extant populations remain in a captive-breeding program at UC Davis and in a fish hatchery operated by the U.S. Fish and Wildlife Service near Shasta Dam.
