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NATURAL HISTORY

DELTA SMELT } Hypomesus transpacificus
FAMILY: Osmeridae

DESCRIPTION: The delta smelt is a small fish, less than three inches in length for most adults. It is nearly translucent, with a steely-blue sheen on the sides.

HABITAT: Delta smelt can tolerate a wide range of salinity and temperatures, but are generally found in brackish water below 25 degrees Celsius. Shallow, fresh or slightly brackish backwater sloughs and edgewaters with good water quality and substrate are used for spawning. Larvae and juveniles need shallow, food-rich nursery habitat. Adequate flow and suitable water quality is required for adult access to spawning habitat and transport of juveniles to Bay rearing habitat.

RANGE: Delta smelt are restricted to the upper reaches of the San Francisco Bay and Sacramento-San Joaquin Delta Estuary, from San Pablo Bay upstream to Sacramento on the Sacramento River and Mossdale on the San Joaquin River.

MIGRATION: In late fall and early winter, maturing adults begin their diffuse upstream migration to freshwater spawning areas. After eggs hatch, larval and juvenile fish move or are transported by flow and tides downstream from freshwater areas in the upper Delta to the brackish low-salinity zone, where they rear for the summer and fall.

BREEDING: Spawning occurs in freshwater during the winter and spring at low water temperatures. Most females produce fewer than 3,000 eggs.

LIFE CYCLE: Delta smelt generally live only one year, although a small percentage of the population may sometimes survive to two years and spawn in one or both years.

FEEDING: Small larvae begin feeding on unicellular algae, planktonic animals, and small crustaceans. As they grow, their diet shifts nearly exclusively to copepods (small crustaceans).

THREATS: Increasing water diversions from the Delta, loss of habitat, competition and predation from introduced species, and impaired water quality due to pesticides and other pollutants are driving the smelt extinct.

POPULATION TREND: The delta smelt population declined by more than 80 percent in the early 1980s, leading to state and federal threatened listings. Smelt numbers fluctuated during the 1990s, and then increased following the cessation of drought in 1992. Abundance again declined drastically in 2002, dropping more than 80 percent in just three years. The smelt population in 2005 was the lowest ever measured, just 2.4 percent of what it was when the species was listed in 1993. Numbers of juvenile smelt found in 2007 surveys were the lowest ever recorded by an order of magnitude.

 

Photo by B. Moose Peterson, USFWS