Hypomesus transpacificus 

Scope: Global
Language: English

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Taxonomy [top]

Kingdom Phylum Class Order Family
Animalia Chordata Actinopterygii Osmeriformes Osmeridae

Scientific Name: Hypomesus transpacificus McAllister, 1963
Common Name(s):
English Delta Smelt

Assessment Information [top]

Red List Category & Criteria: Critically Endangered A2bce ver 3.1
Year Published: 2014
Date Assessed: 2012-02-17
Assessor(s): NatureServe
Reviewer(s): Smith, K. & Darwall, W.R.T.
Facilitator/Compiler(s): Hammerson, G.A. & Ormes, M.
This species is listed as Critically Endangered because apparent population size has declined by more than 90 percent over the past 10 years (three generations is around 3 years), and the decline is ongoing. Extent of occurrence is less than 1,000 sq km, area of occupancy is probably less than 500 sq km, and number of locations is one. Population size is unknown. Quantitative analysis calculated a 50 percent likelihood that the species could reach effective extinction within 20 years. Should the recent steep decline abate, the species would still easily qualify for Endangered status.
Previously published Red List assessments:

Geographic Range [top]

Range Description:This fish is endemic to the upper San Francisco Estuary, principally the upper Sacramento-San Joaquin Delta (primarily below Isleton in the Sacramento River [probably formerly north to Sacramento], and below Mossdale in the San Joaquin River) and Suisun Bay (California Department of Fish and Game 1990, Moyle 2002). The species congregates in upper Suisun Bay and occurs in small numbers in larger sloughs of Suisun Marsh when river outflows are high (mainly March to mid-June); in 1994-1998, delta smelt were found mainly in Suisun Bay (Moyle et al. 1989, Moyle 2002). During periods of drought, the centre of abundance has been in the northwestern Delta in the channel of the Sacramento River, and the species is then rare in Suisun Bay (Moyle et al. 1989, Moyle 2002). During years with average or high outflow, smelt may concentrate prior to spawning movements anywhere from the Sacramento River near Decker Island to Suisun Bay (Moyle 2002). Spawning occurs in the sloughs and channels of the upper Delta (e.g., Barker, Lindsey, Cache, Georgiana, Prospect, Beaver, Hog, and Sycamore sloughs) and in the Sacramento River above Rio Vista (Moyle 2002); in some years spawning also occurs north of Suisun Bay in Montezuma and Suisun sloughs and their tributaries or in the Napa River estuary (Federal Register, 6 January 1994; Moyle 2002). It is unclear if smelt in the Napa River are self perpetuating or if frequent recolonization from the Delta is necessary to maintain a population there (USFWS 2004). Larvae have been recorded from the Sacramento River as far north as its confluence with the Feather River (Wang 1991, cited in Federal Register 6 January 1994). Sometimes individuals are washed into San Pablo Bay, but permanent populations do not exist there (Moyle 2002).
Countries occurrence:
United States
Additional data:
Range Map:Click here to open the map viewer and explore range.

Population [top]

Population:This species is represented by what can be regarded as one large occurrence (e.g., see Trenham et al. 1998).

Total adult population size is unknown but is much larger than 10,000. Population size fluctuates and is very difficult to estimate (Moyle 2002).

Historically, this was the most abundant pelagic fish in the upper Sacramento-San Joaquin estuary (Erikkila et al. 1950), and it remained abundant in California Department of Fish and Game trawl surveys in the Delta during the early 1970s (Stevens and Miller 1983, Moyle et al. 1989, Stevens et al. 1990). Now the species is much less numerous. In the early 1990s, the delta smelt was under a high degree of threat from the severe 1987-1992 drought. The species persisted in small numbers and rebounded to pre-decline levels in 1993, suggesting that recovery potential is fairly high. However, the subsequent decline in 1994, a critical water year, to a then all-time low annual abundance index, illustrates the high degree of threat that neutralizes gains in abundance that result from good water years. USFWS (2004) concluded that although there has been a general increase in numbers since the low point during a long period of drought in the 1980s and early 1990s, delta smelt abundance has not recovered to its pre-decline (prior to 1982) levels and that the overall trend is negative. USFWS (2010) found a continuing steep decline.

Trend over the past 10 years or three generations (three generations is a shorter time period than 10 years) has been a steep decline. Apparent abundance has continued to decline since the significant decline that occurred in 2002; the most recent fall mid-water trawl abundance index (2009) was the lowest ever recorded (less than one-tenth the level of 2003) (USFWS 2010). A 2005 population viability analysis calculated a 50 percent likelihood that the species could reach effective extinction within 20 years (see USFWS 5-year review summarized in 2010).
Current Population Trend:Decreasing
Additional data:

Habitat and Ecology [top]

Habitat and Ecology:This euryhaline species inhabits open waters of bays, tidal rivers, channels, and sloughs; it rarely occurs in water with salinity of more than 10-12 ppt; when not spawning, it tends to concentrate where salt water and freshwater mix (salinity about 2 ppt) and zooplankton populations are dense (Moyle et al. 1989; USFWS 1993, 1996). Populations are concentrated mainly in the lower Delta and upper Suisun Bay after breeding (at least formerly). Adequate freshwater flows are needed to transport young to rearing habitat and to maintain rearing habitat in a favourable location (i.e., Suisun Bay).

Spawning occurs in freshwater (sometimes in slightly brackish water), primarily in tidal dead-end sloughs and channel edgewaters. Eggs are demersal and adhesive. Larvae are buoyant and are carried downstream until they reach the mixing zone; area of the 2 ppt isohaline is an important rearing habitat for the young. See USFWS (1996) and Moyle (2002).
Generation Length (years):1
Movement patterns:Not a Migrant

Threats [top]

Major Threat(s): The decline may be attributable to the Delta Smelt's low fecundity and one-year lifespan, in combination with changes and fluctuations in the estuarine ecosystem. These changes and fluctuations include: reduction in outflows (water projects have allowed the intrusion of higher salinity water into the Delta, restricting the smelt's spawning and nursery areas to less favourable river channel habitat and making them more vulnerable to diversion in massive water project pumping plants); entrainment losses of larvae and adults to water diversion (attempts to rescue entrained smelt are probably largely unsuccessful, as these fishes are relatively fragile and often die when trapped and transported); high outflows during years with unusually high rainfall (flush smelt and their food resources out of the system; may make it easier for exotic species of copepods, clams and fishes to colonize); and changes in food resources (e.g., increased abundance of various species of exotic copepods, which could deplete smelt food resources and/or be more difficult for smelt to capture; the introduced clam, Potamocorbula, now abundant in Suisun Bay, may reduce availability of zooplankton to smelt larvae; the diatom Melosira, which is difficult for zooplankton to consume, sometimes becomes very abundant in the Delta) (Moyle et al. 1989; Moyle 2002; USFWS 1993, 1996).

Hybridization (e.g., introduced Hypomesus nipponensis represents a potential threat through hybridization and/or competition; hybrids may be sterile but interbreeding may result in loss of Delta Smelt gametes).

Water pollution (toxic substances from agriculture and urbanization) poses a threat of unknown degree (California Department of Fish and Game 1990, Moyle 2002).

USFWS (Federal Register, 6 January 1994) listed the following activities as potential threats to critical habitat: sand and gravel excavation in river channels or marshes; diking wetlands for conversion to farmland and dredging to maintain these dikes; levee maintenance and bank-protection activities, such as riprapping, removal of vegetation, and placement of dredged materials on levees or banks; operation of the Montezuma Slough Control Structure; and bridge and marina construction.

In reviewing the status of the Delta Smelt, USFWS (2004) made the following conclusions. Threats posed by extreme outflow conditions, the operations of the state and federal water projects, and other water diversions as described in the original listing (USFWS 1993) remain. Although some conservation measures have helped to ameliorate threats from adverse hydrological conditions and water diversions, it is unclear how effective these will continue to be over time based on available funding and future demands for water. Increases in water demands are likely to result in less suitable rearing conditions for Delta Smelt in Suisun Marsh, increased vulnerability to entrainment, and less water available for maintaining favourable habitat conditions. The importance of exposure to toxic chemicals on the population of Delta Smelt is highly uncertain. Threats posed by disease and predation have not been sufficiently studied, and their effects on Delta Smelt remain poorly known. Hybridization with Hypomesus nipponensis appears not to be a major threat. Existing regulatory mechanisms, particularly relating to water management and its affect on smelt habitat, are inadequate to assure the long-term existence of Delta Smelt in Suisun Bay and the Delta (USFWS 2004). The only new information concerning the Delta Smelt's population size and extinction probability indicates that the population is at risk of falling below an effective population size and therefore in danger of becoming extinct.

An updated status review (USFWS 2010) yielded the following conclusions: Destruction, modification, or curtailment of habitat poses a current and future threat. Operation of upstream reservoirs, increased water exports, and upstream diversions have altered the location and extent of the low-salinity zone, concentrating smelt in an area with competing fish species. Upstream reservoirs and the increased presence of the exotic submerged aquatic plant Egeria densa have also reduced turbidity levels in rearing habitat, which may reduce foraging efficiency. Predation is probably a low-level threat at this time. The continued decline in apparent abundance suggests that existing regulatory mechanisms, as currently implemented, are not adequate to reduce the threats to the species. Other threats includes entrainment by water export facilities and into power plant diversions, contaminants, and small population effects.

Conservation Actions [top]

Conservation Actions: The primary conservation need is recovery of natural processes in the Sacramento-San Francisco estuary, including outflow (Moyle 2002).

Moyle et al. (1989) emphasized that present mechanisms that regulate freshwater flow through the Delta have been inadequate to protect Delta Smelt. They concluded that the key conservation action needed is to provide enough outflow through the Delta to ensure that the entrapment zone is located in Suisun Bay during March-May in all but the driest years; this zone should not be located outside Suisun Bay for more than two successive years; such a flow regime would also benefit other species such as the striped bass. See recovery plan (USFWS 1996).

Citation: NatureServe. 2014. Hypomesus transpacificus. The IUCN Red List of Threatened Species 2014: e.T10722A18229095. . Downloaded on 14 August 2018.
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