The delta smelt (Hypomesus transpacificus) is an endangered slender-bodied smelt, about 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).Sommer, T. et al. 2011. The spawning migration of delta smelt in the Upper San Francisco Estuary. San Francisco Estuary and Watershed Science: 9(2). It functions as an indicator species for the overall health of the Delta's ecosystem. Delta Smelt are usually found at temperatures of less than 25 °C and prefer temperatures of around 20 °C. They are euryhaline but occur mostly at salinities of 0–7 practical salinity units.
Because of its one-year lifecycle and relatively low fecundity, it is very susceptible to changes in the environmental conditions of its native habitat.Moyle, PB. 2002. Inland fishes of California. University of California Press, Berkeley. 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.
Delta smelt is easily confused with longfin smelt (Spirinchus thaleichthys). They are both small, pelagic fish species native to the estuarine and coastal waters of the Pacific coast, but they exhibit distinct ecological, morphological, and behavioral characteristics. The delta smelt, a key indicator species, is endemic to the San Francisco Bay-Delta and has a slender body averaging 5–7 cm in length, with a translucent appearance and a faint, blue lateral line. It primarily inhabits low-salinity waters and is critically endangered due to habitat loss and water management practices. In contrast, the longfin smelt is slightly larger, typically reaching up to 15 cm, and is distinguished by its long pectoral fins that extend beyond the base of the pelvic fins.
Taxonomy and evolution
The delta smelt is one of five currently recognized species within the genus Hypomesus, which is part of the larger smelt family, Osmeridae. 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 family Osmeridae was re-examined 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 sister to the remaining species of Hypomesus, the most recent speciation event in Hypomesus is known to have been 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.
Genetic studies of delta smelt populations in recent years have revealed potential signs of adaptation to environmental changes, such as increased tolerance to warmer water temperatures. These changes are thought to be driven by the increasingly challenging conditions in the San Francisco Estuary, where rapid alterations in water temperature, salinity, and habitat availability have exerted selective pressures on the species. Evidence of genetic differentiation between wild and hatchery populations has also been observed, raising concerns about the long-term viability of hatchery supplementation programs designed to conserve the species.
Habitat 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.Delta smelt populations tend to aggregate in areas with moderate turbidity, which enhances their ability to feed and avoid predators.
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.Radtke, LD (1966) Distribution of smelt, juvenile sturgeon and starry flounder in the Sacramento-San Joaquin Delta. In: Fish bulletin. California Department of Fish and Game, Sacramento, pp 115–129 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.CDFG (2003) Comment letter on the five-year status review of the delta smelt. California Department of Fish and Game, California, p 6Miller J, Swanson C, Poole KS (2006) Emergency petition to list the delta smelt (Hypomesus transpacificus) as an endangered species under the Endangered Species Act. Center for Biological Diversity, Bay Institute, & Natural Resources Defense Council. 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.Bennett, WA (2005) Critical assessment of the delta smelt population in the San Francisco Estuary, California. San Francisco Estuary Watershed Sci 3:1–71.
Lifecycle
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.
Endangered status Historically, delta smelt were relatively abundant in the upper Sacramento-San Joaquin Estuary, with populations declining dramatically in the 1980s.Newman KB (2008) Sample design-based methodology for estimating delta smelt abundance. San Francisco Estuary Watershed Sci 6:1–18. 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.CDFG (2010) State & federally listed endangered & threatened animals of California. California Department of Fish & Game, State of California, The Natural Resources Agency, California.Federal Register 58:12863; March 5, 1993 Critical habitat was listed for delta smelt on December 19, 1994.Federal Register 59:65256
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.Nichols FH, Cloern JE, Luoma SN, Peterson DH (1986) The modification of an estuary. Science 231:567–573. 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.
Environmental threats and challenges The decline of the delta smelt may have been caused by environmental challenges, both natural and human-derived.
Habitat degradation The transformation of the Sacramento-San Joaquin Delta into a network of levees and channels changed the natural hydrology of the region. This has resulted in the loss of important tidal wetlands and shallow water habitats that are essential for delta smelt spawning, feeding, and rearing. Agricultural and urban developments have dramatically reduced the available natural habitat for the species, pushing delta smelt into fragmented and degraded environments. Water diversion projects, such as the Central Valley Project and the State Water Project, have further reduced the availability of freshwater flows, particularly during dry years. These reduced flows impact not only the quality of delta smelt habitat but also their access to low-salinity areas, which are critical for survival.
Invasive species The introduction of non-native species, such as the overbite clam (Potamocorbula amurensis) and Mississippi silverside (Menidia audens), has greatly affected delta smelt survival. These invasive species compete with delta smelt for food resources, particularly zooplankton, which is crucial for their diet. Invasive predators, such as striped bass and largemouth bass, also pose a significant threat by preying on delta smelt, further reducing their already dwindling populations. Invasive aquatic plants, like water hyacinth (Eichhornia crassipes), have altered the structure of the delta ecosystem by creating dense mats of vegetation. These mats limit water movement and light penetration, reducing water quality and making the delta less hospitable for delta smelt and other native species.
Water contamination