Delta smelt, Hypomesus transpacificus, is an endangered slender-bodied smelt, about 5 to 7 cm (2 to 2.8 in) long, of the Osmeridae family. Endemic to the upper Sacramento-San Joaquin estuary of California

California

California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

, it mainly inhabits the freshwater-saltwater mixing zone of the estuary, except during its spawning

Spawn (biology)

Spawn refers to the eggs and sperm released or deposited, usually into water, by aquatic animals. As a verb, spawn refers to the process of releasing the eggs and sperm, also called spawning...

 season, which primarily takes place during the early spring

Spring (season)

Spring is one of the four temperate seasons, the transition period between winter and summer. Spring and "springtime" refer to the season, and broadly to ideas of rebirth, renewal and regrowth. The specific definition of the exact timing of "spring" varies according to local climate, cultures and...

 months from March until May.

Because of its one-year life cycle and relatively low fecundity

Fecundity

Fecundity, derived from the word fecund, generally refers to the ability to reproduce. In demography, fecundity is the potential reproductive capacity of an individual or population. In biology, the definition is more equivalent to fertility, or the actual reproductive rate of an organism or...

, it is very susceptible to changes in the environmental conditions of its native habitat.