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.

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.

[10] 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.

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.

Young delta smelt then feed and grow in the mixing zone before starting their upstream spawning migration in late fall or early winter.

[25] 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.

Agricultural and urban developments have dramatically reduced the available natural habitat for the species, pushing delta smelt into fragmented and degraded environments.

[27] The introduction of non-native species, such as the overbite clam(Potamocorbula amurensis) and Mississippi silverside (Menidia audens), has greatly affected delta smelt survival.

Pesticide use, particularly in agricultural areas surrounding the delta, has led to toxic runoff entering the waterways, affecting the smelt's reproductive success and overall health.

[28] Histopathological assessments conducted over several year-classes of delta smelt have revealed significant tissue damage, particularly in the liver and gills, which is linked to chronic exposure to pollutants such as ammonia and heavy metals.

Rising water temperatures, changes in precipitation patterns, and increased frequency of droughts are expected to further reduce the availability of suitable habitat for delta smelt.

The diversion of freshwater for agriculture, combined with increasing urban water demands, has left the delta smelt with fewer areas of low salinity where they can thrive.

[33] The Natural Resources Defense Council sued, and in 2007, Fresno U.S. District Court Judge Oliver Wanger found the biological opinion was arbitrary and capricious and ordered protections for the delta smelt while the document was redone.

[36] When six new plaintiffs sued, Judge Wanger preliminarily ordered the FWS to give him weekly justifications of delta flow restrictions and appointed four scientists as his own expert witnesses.

[28] Efforts to conserve and restore the delta smelt (Hypomesus transpacificus) population have been ongoing for decades, as this species continues to face significant threats from habitat degradation, climate change, and water management practices.

Multiple state and federal agencies, in partnership with universities and conservation organizations, have implemented a variety of strategies aimed at preventing the extinction of this critically endangered species.

These restored habitats aim to provide the delta smelt with improved spawning grounds, adequate food resources, and reduced predation pressures.

[15] However, the success of these restoration efforts is often challenged by the ongoing presence of invasive species, such as the overbite clam (Potamocorbula amurensis) and the Mississippi silverside (Menidia audens), which compete with delta smelt for food and further reduce their chances of survival.

Researchers have conducted extensive studies on the impact of environmental factors such as temperature, salinity, and turbidity on delta smelt behavior, growth, and survival.

Recent studies have highlighted how climate change, including rising water temperatures and altered precipitation patterns, is likely to exacerbate the challenges faced by delta smelt populations.

[30] Furthermore, the use of advanced modeling techniques has helped predict how changes in water flow and habitat conditions will affect delta smelt distribution and survival, providing essential data for future conservation efforts.