The delta smelt is a type of fish closely related to the salmon and the trout. This smelt inhabits the San Francisco Bay Delta, living exclusively in northern California waterways. Before 1980, they were one of the most abundant fish in the area. Since then, their population has continuously declined. The scientific name for the delta smelt is hypomesus transpacificus.
As small fish, delta smelts are 2.4–2.7 inches (6–7 cm) long and have narrow bodies. They have small mouths, with sharp tiny teeth in their upper and lower jaws. Their eyes are large, and, when living, their silvery bodies seem almost translucent.
Delta smelts prefer brackish water, though they move into fresh water to spawn. They can be found in estuaries from the San Francisco Bay Delta to the Sacramento-San Joaquin Delta. They usually live in shallow water, near the surface, and are eurythermal, meaning they can tolerate a variety of temperatures. They eat primarily zooplankton.
Spawning in fresh water, the delta smelt moves upstream into the nearby rivers during mating season. The spawning season varies, beginning sometimes as early as December and ending as late as July. The larger, more mature fish make the trip first followed by the smaller fish later in the year. Water temperatures in spawning areas need to be between 53.6–64.4°F (12–18°C) for spawning to be successful.
Delta smelt eggs are adhesive and usually attached to rocks or aquatic plants. Eggs hatch after eight to 14 days into tiny larvae, only measuring about 0.2 inches (5 mm) long. While they grow into juvenile fish, the larvae are transported downriver by the river's currents. This type of smelt is an annual fish, which means its entire lifespan occurs in a single year. There is, however, some indication that the fish occasionally lives as long as two years, possibly reproducing in both years.
Of the 29 indigenous fish species found in the San Francisco Bay area, 12 species, including the delta smelt, have been driven to near extinction. The delta smelt has been on the endangered species list since 1993. Severe pollution — at sometimes toxic levels — as well as the introduction of new species and man-made water diversions have led to this fish's threatened status. The species is continuing to decline despite measures taken for their protection. In 2007, too few were found to estimate the then current population of the species, and scientist project the delta smelt may be extinct by 2026 unless more serious measures are taken.