Delta Smelt

Hypomesus transpacificus

Description

The delta smelt is a slender-bodied fish typically 2.4-2.8 in (6.1-7.1 cm) long. Some individuals may reach a standard length of 4.7 in (11.9 cm). Live specimens are nearly translucent and have a steely-blue sheen to their sides. Occasionally there may be one chromatophore between the mandibles, but usually none are present. Its mouth is small, with a maxilla that does not extend past the mid-point of the eye. The eyes are relatively large; the orbit width is contained about 3.5-4 times in the head length. Small, pointed teeth are present in the upper and lower jaws. The first gill arch has 27-33 gill rakers and there are seven branchiostegal rays. There are nine to ten dorsal fin rays, eight pelvic fin rays, ten to 12 pectoral fin rays, and 15-17 anal fin rays. The lateral line is incomplete and has 53-60 scales along it. There are four to five pyloric caeca.

Behavior

Delta smelt inhabit open, surface waters of the Delta and Suisun Bay, where they presumably school. Spawning takes place between January and July, as inferred from larvae collected during this period. Timing and length of the spawning season may vary. Spawning usually takes place from late March through mid-May in tow outflow years.

Spawning pulses have not been detected. Spawning occurs in sloughs and shallow edge-waters of channels in the upper Delta.

Laboratory observations have indicated that delta smelt are broadcast spawners and that the eggs sink to the bottom and stick to hard substrates such as: rock, gravel, tree roots or submerged branches, and submerged vegetation. At 57-61°F (14-16°C), embryonic development to hatching takes nine to 14 days and feeding begins four to five days later.

Newly hatched delta smelt have a large oil globule that makes them semi-buoyant, allowing them to maintain themselves just off the bottom, where they feed on microscopic crustaceans and other microscopic prey. Once the swimbladder (a gas-filled organ that allows fish to maintain neutral buoyancy) develops, larvae become more buoyant and rise up higher into the water column. At this stage, most are presumably washed downstream until they reach the mixing zone or the area immediately upstream of it.

Growth is rapid and juvenile fish are 1.6-2 in (4-5 cm) long by early August. By this time, young-of-year fish dominate trawl catches of delta smelt, and adults become rare. Delta smelt reach 2.2-2.8 in (5.6-7.1 cm) standard length in seven to nine months. Growth during the next three months slows down considerably, presumably because most of the energy ingested is being directed towards gonadal development. There is no correlation between size and fecundity, and females between 2.3-2.8 in (5.8-7.1 cm) standard length lay 1,200-2,600 eggs, which is low when compared to two other species of Osmeridae occurring in California that exhibit fecundities from 5,000 to 25,000 eggs per female. The abrupt change from a single-age, adult cohort during spawning in spring to a population dominated by juveniles in summer suggests strongly that most adults die after they spawn.

The primary foods for all life stages of the delta smelt are nauplius, copepodite, copepodid, and adult stages of the euryhaline copepod Eurytemora affinis. Pelagic larvae are zooplanktivores and feed on copepods, cladocerans, and amphipods. Adults consume E. affinis during all times of the year. The opossum shrimp (Mysis relicta ) is secondarily important as food for adults, and cladocerans are consumed seasonally.

Habitat

H. transpacificus is the only smelt endemic to California and the only true native estuarine species found in the Sacramento-San Joaquin estuary, known as the Delta. It is a euryhaline species, a species adapted to living in fresh and brackish water, that occupies estuarine areas with salinities below 2 grams per liter. It rarely occurs in estuarine waters with more than 10-12 ppt salinity, about one-third sea water.

Delta smelt is adapted for life in the mixing zone of the Sacramento-San Joaquin estuary. The estuary is an ecosystem where the mixing zone and salinity levels are determined by the interaction of river out-flow and tidal action. Individuals appear to be most abundant in shallow, low salinity water associated with the mixing zone, except when they spawn.

The larvae require the high microzooplankton densities produced by the mixing zone environment. The best survival and growth of larvae occurs when the mixing zone occupies a large geographic area, including extensive shoal regions that provide suitable spawning substrates within the euphotic zone (depths less than 13 ft, or 4 m).

During periods of drought and increased water diversions, the mixing zone and associated delta smelt populations are shifted farther upstream in the Delta. The mixing zone is now primarily located in river channels during the entire year because of increased water exports and diversions. When located upstream, the mixing zone becomes confined to the deep river channels, becomes smaller in total surface area, contains very few shoal areas of suitable spawning substrates, may have swifter, more turbulent water currents, and lacks high zooplankton productivity. Reproduction of the species is very likely adversely affected now that the mixing zone is located in the main channels of the Delta.

Distribution

The delta smelt is endemic to California and the only true native estuarine species found in the Delta. Historically, the fish occurred from Suisun Bay upstream into the Sacramento and San Joaquin Rivers. The species historically congregated in upper Suisun Bay and Montezuma Slough when the Sacramento and San Joaquin river flows were high. During very high river outflows, some individuals would be washed into San Pablo Bay.

The species is now rare in Suisun Bay, and virtually absent from Suisun Marsh where it was once seasonally common. The center of the species abundance has shifted to the Sacramento River channel in the Delta. They occur in the Delta primarily below Isle-ton on the Sacramento River, below Mossdale on the San Joaquin River, and in Suisun Bay. They move into freshwater when spawning (ranging from January to July) and can occur in: (1) the Sacramento River as high as Sacramento, (2) the Mokelumne River system, (3) the Cache Slough region, (4) the Delta, and, (5) the Montezuma Slough area of the estuary. During high outflow periods, they may be washed into San Pablo Bay, but they do not establish permanent populations there. Since 1982, the center of delta smelt abundance has been the northwestern Delta in the channel of the Sacramento River. However, high outflows in the winter of 1992-93 allowed delta smelt to recolonize Suisun Bay in 1993. Delta smelt are captured seasonally in Suisun Marsh.

Delta smelt were once one of the most common pelagic (living in open water away from the bottom) fish in the upper Sacramento-San Joaquin estuary. Delta smelt abundance from year to year has fluctuated greatly in the past, but between 1982 and 1992 their population was consistently low. The decline became precipitous in 1982 and 1983 due to extremely high outflows and continued through the drought years 1987-92. In 1993, numbers increased considerably, apparently in response to a wet winter and spring. During the period 1982-92, most of the population was confined to the Sacramento River channel between Collinsville and Rio Vista. This was still an area of high abundance in 1993, but delta smelt were also abundant in Suisun Bay. The actual size of the delta smelt population is not known. In 1990, the estimated population was 280,000. The pelagic life style of delta smelt, short life span, spawning habits, and relatively low fecundity indicate that a fairly substantial population probably is necessary to keep the species from becoming extinct.

Threats

The delta smelt occurs only in Suisun Bay and the Sacramento-San Joaquin estuary near San Francisco Bay in California. The species has declined nearly 90% over the last 20 years, and is primarily threatened by large freshwater exports from the Sacramento River and San Joaquin River diversions for agriculture and urban use. Prolonged drought, introduced nonindigenous aquatic species, reduction in abundance of key food organisms, and agricultural and industrial chemicals also threaten this species.

The decline in the species' population was concurrent with increased human changes to seasonal Delta hydrology, freshwater exports, and the accompanying changes in the temporal, spatial, and relative ratios of water diversions. Delta water diversions and exports presently total up to about nine million acre-feet per year. State and Federal projects presently export about six million acre-feet per year when there is sufficient water available, and in-Delta agricultural uses result in diversion of about three million additional acre-feet per year. The U. S. Fish & Wildlife Service is currently aware of another 21 major Central Valley Project, State Water Project, or private organizations' proposals that will result in increased water exports from the Delta. This will reduce water inflow to the Delta, changing the timing and volume of Delta inflow, or increasing heavy metal contamination into the Delta.

Since 1983, the proportion of water exported from the Delta during October through March has been higher than in earlier years. The timing of these proportionally higher exports have coincided with the species' spawning season. During periods of high export pumping and low to moderate river outflows, reaches of the San Joaquin River reverse direction and flow to the pumping plants located in the southern Delta. During this reversal flow, out-migrating larval and juvenile fish of many species become disoriented. Large mortalities occur as a result of entrainment and predation by striped bass at the various pumping plants and other water diversion sites. Net positive riverine flows and estuarine outflows of sufficient magnitude are required for H. transpacificus larvae to be carried downstream into the upper end of the mixing zone of the estuary rather than upstream to the pumping plants.

During periods of drought and increased water diversions, the mixing zone and associated delta smelt populations are shifted farther upstream in the Delta. When located upstream, the mixing zone becomes confined to the deep river channels, becomes smaller in total surface area, contains very few shoal areas of suitable spawning substrates, may have swifter, more turbulent water currents, and lacks high zooplankton productivity.

Harvesting of the species is not known to be a major limiting factor, though some individuals are harvested as a non-target by-catch in commercial bait fisheries for other baitfish species. Predation by striped bass, as mentioned above, has become a limiting factor coupled with the reversal of flow in riverine habitats.

Poor water quality may also be limiting factor. All major rivers in the species' range are exposed to large volumes of agricultural and industrial chemicals that are applied in the California Central Valley watersheds. Agricultural chemicals and their residues, and chemicals originating in urban runoff, find their way into the rivers and estuaries. In the Colusa Basin Drainage Canal, significant toxicity has been documented in striped bass embryos and larvae, medaka larvae, the major food organism of the striped bass larvae and juveniles, and the opossum shrimp. Some heavy metal contaminants have been released into the Delta from industrial and mining enterprises. These compounds could adversely affect the species' survival.

Nonindigenous aquatic species have been introduced into the Delta by untreated discharges of ship ballast water. Several introduced species adversely affect the delta smelt. An Asian clam, introduced as larvae at the beginning of the present drought, was first discovered in the Suisun Bay in October of 1986. By June of 1987, the Asian clam was nearly everywhere in Suisun, San Pablo, and San Francisco Bays irrespective of salinity, water depth, and sediment type at densities greater than 10,000 individuals per square meter. This clam could potentially play an important role in affecting the phytoplankton dynamics in the estuary. It may have an affect on the fish by decreasing phytoplankton and by directly consuming the smelt's primary food. Weakened larvae due to reduced food availability or feeding efficiency causing decreased food ingestion rates makes the species more vulnerable to starvation or predation.

Water diversions throughout the species' range has been the major cause of the species decline. Future diversions must be carefully considered so as not to further jeopardize the species. Stricter regulation on chemical runoff into the Delta region needs to be established. Discharges of ship ballast water must be properly treated to inhibit further introduction of nonindigenous species into the habitat. Stocking of the area with striped bass should be discontinued due to the increased predation threat caused by this species. Reintroduction of E. affinis, the species' main food source, should be instigated to lessen the chance of the smelt's starvation. Captive breeding programs for the species should be implemented to increase the genetic variance and increase the species' chance for survival.

The waters of the estuary receive a variety of toxic substances, including agricultural pesticides, heavy metals, and other products of urbanized society. The effects of these toxic compounds on larval fishes and their food supply are poorly known, but there is growing evidence that larval striped bass are suffering direct mortality or additional stress from low concentrations of toxic substances. There is also evidence that planktonic organisms upon which delta smelt feed may be depleted on occasion by brief aperiodic flushing of high concentrations of pesticides through the system. It is not known if these substances also are affecting delta smelt.

Although delta smelt has managed to coexist with introduced bass and other competitors in the past, it is quite possible that at low population levels interactions with them could prevent recovery. In particular, inland silversides are usually collected in areas where delta smelt may spawn and they could be major predators on eggs and larvae. Since 1988, chameleon gobies have increased dramatically in the Delta, and adults of this species and yellowfin goby may prey on delta smelt eggs and larvae.

In past years, efforts to enhance striped bass populations by planting large numbers of juveniles from hatcheries could have had a negative effect on other pelagic fishes in the estuary once bass have reached a size where predation begins on these fish. The enhanced predator populations, without a concomitant enhancement of prey populations such as delta smelt, may have resulted in excessive predation pressure on prey species. In 1992, planting of juvenile striped bass was halted indefinitely because of potential effects of predation on juvenile winter-run chinook salmon.

Conservation and Recovery

Delta smelt adults seek shallow, fresh or slightly brackish backwater sloughs and edgewaters for spawning. To ensure egg hatching and larval viability, spawning areas also must provide water with low concentrations of pollutants, and adequate substrates for egg attachment comprised of submerged tree roots and branches and submersed vegetation.

To ensure that delta smelt larvae are transported from the area where they are hatched to shallow, productive rearing or nursery habitat, the Sacramento and San Joaquin Rivers and their tributary channels must be protected from physical disturbance, such as sand and gravel mining, diking, dredging, and levee or bank protection and maintenance, and flow disruption. Adequate river flow is necessary to transport larvae from upstream spawning areas to rearing habitat in Suisun Bay.

Delta smelt larvae and juveniles must be provided a shallow, protective, food-rich environment in which to protect larval, juvenile, and adult delta smelt from entrainment in the water projects. Protection of rearing habitat conditions may be required from the beginning of February through the summer.

Adult delta smelt must be provided unrestricted access to suitable spawning habitat in a period that may extend from December to July. Adequate flow and suitable water quality may need to be maintained to attract migrating adults in the Sacramento and San Joaquin River channels and their associated tributaries. These areas should be protected from physical disturbance and flow disruption during migratory periods.

Delta smelt culture techniques and facilities are being developed. Initial efforts to breed delta smelt in captivity have been successful, although rearing beyond the larval stage so far has not been possible. However, if hatchery propagation is to be successful, fish must be released into an environment that provides ample food, low levels of toxic compounds, and low entrainment losses.

Contact

U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N. E. 11th Ave.
Portland, Oregon 97232-4181
Telephone: (503) 231-6121
http://pacific.fws.gov/

Reference

U.S. Fish and Wildlife Service. 5 March 1993. "Endangered and Threatened Wildlife and Plants; Determination of Threatened Status for the Delta Smelt." Federal Register 58 (42): 12854-12863.