Delta smelt, Hypomesus transpacificus, is an endangered slender-bodied smelt, about 5 to 7 cm (2.0 to 2.8 in) long, in the Osmeridae family. 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 freshwater following winter "first flush" flow events (around March to May).
Because of its one-year lifecycle and relatively low fecundity, it is very susceptible to changes in the environmental conditions of its native habitat. Efforts to protect the endangered fish from further decline have focused on limiting or modifying the large-scale pumping activities of state and federal water projects at the southern end of the estuary.
Taxonomy and evolution
|Phylogeny of the genus Hypomesus.|
The delta smelt is one of five currently recognized species within the Hypomesus genus, which is part of the larger Osmeridae family of smelts. The genus has been subject to many revisions since it was first classified by Gill in 1863. The first major revision occurred in 1963, when the Osmeridae family was reexamined by Canadian ichthyologist Donald Evan McAllister. Expanding on Japanese researcher Hamada's earlier determination that H. olidus was not a monolithic widespread species, but rather one of three distinct species of Hypomesus, McAllister assigned them new names, and further delineated what he believed were four subspecies. This was the first description of H. transpacificus; named for its supposed occurrence on both sides of the Pacific, and also "to the friendship of Japanese and Canadian ichthyologists." He separated these geographically isolated populations into separate subspecies: H. t. transpacificus and H. t. nipponensis.
Modern analysis of the genus would elevate all of McAllister's subspecies to full species status, based on fin ray counts and the number of chromatophores between their mandibles, a change which genetic analysis has supported. In fact, genetic analysis would conclude that despite their morphological similarities, H. nipponensis and H. transpacificus are actually members of different phylogenetic clades.
The abbreviated distribution of Hypomesus species along both the east and west sides of the Pacific ocean suggests that their common ancestor had a range that would have crossed the Pacific. Researchers have hypothesized that climatic changes may have reduced the range of the ancestral species during cooling periods, which would have created a reproductive barrier, allowing speciation to occur. Although the low number of species in the genus and high levels of homoplasy have frustrated attempts to determine whether the northern Pacific H. olidus or H. nipponensis are the basal species of Hypomesus, it is known that the most recent speciation event in Hypomesus was between the two native east Pacific species, H. pretiosus and H. transpacificus. This is plausibly due to a geographic isolation of a widespread eastern Pacific ancestor, of which some members were isolated in a freshwater basin in western California, possibly in the lakes that would have been located in the southern San Joaquin Valley during the Pleistocene epoch.
The 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, even 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. It is likely that water export operations 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. Delta smelt spawning occurs in spring in river channels and tidally influenced backwater sloughs upstream of the mixing zone where saltwater meets freshwater. 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.
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.
On August 31, 2007, Federal Judge Oliver Wanger of the United States District Court for the Eastern District of California protected the delta smelt by severely curtailing human-use water deliveries from the San Joaquin-Sacramento Delta from December to June. These are the pumps at the Banks Pumping Plant that send water through the California Aqueduct to Central and Southern California for agricultural and residential use. Judge Wanger's later accusations that federal scientists had not provided complete or accurate information were investigated by RESOLVE and shown to be unfounded.
Although allegations have been made that this protection has hurt California's agricultural sector, with the devastation of hundreds of thousands of acres of farmland and the loss of tens of thousands of jobs in the Central Valley, a 2009 United States Department of Agriculture study estimated that job losses due to smelt protection were closer to 5,000. An additional 16,000 job losses in the Central Valley were attributed to the drought California had been experiencing in recent years.
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