Federal Listing - Threatened (1993)

State Listing - Threatened (1993)

General Habitat: Sacramento-San Joaquin Estuary

The delta smelt is a small, slender-bodied fish, with a typical adult size of 2-3 inches (55- 70mm standard length) although some may reach lengths up to 5 inches (130mm). Live delta smelt have a steel blue sheen on the lateral sides and appear somewhat translucent. Like other members of the family Osmeridae, the delta smelt has an adipose fin (a small fleshy fin on the back between the dorsal fin and tail). Delta smelt are found only in the Sacramento-San Joaquin Estuary (the area where the Sacramento and San Joaquin Rivers flow into San Francisco Bay. They have been found as far upstream as the mouth of the American River on the Sacramento River and Mossdale on the San Joaquin River. They extend downstream as far as San Pablo Bay. Delta smelt are found in brackish water. They usually inhabit salinity ranges of less than 2 parts per thousand (ppt) and are rarely found at salinities greater than 14ppt.

During the late winter to early summer, delta smelt migrate to freshwater to spawn. Females only produce between 1000 and 2600 eggs which sink to the bottom and attach to the substrate. Larvae hatch between 10-14 days, are planktonic (float with the water currents), and are washed downstream until they reach areas near the entrapment zone where salt and fresh water mix. Delta smelt are fast growing and shortlived with the majority of growth within the first 7 to 9 months of life. Most smelt die after spawning in the early spring although a few survive to a second year. Delta smelt feed entirely on small crustaceans called zooplankton.

The delta smelt historically was one of the most common fish in the Sacramento-San Joaquin Estuary. Delta smelt abundance fluctuates greatly from year to year, however, information from seven independent data sets demonstrated a dramatic decline of the delta smelt population and low population levels from 1983 to 1992. In 1993 abundance increased in an apparent response to an increase in available habitat brought about by a wet winter and spring. Fall abundance of delta smelt is usually higher when low salinities of 2ppt or less occur in Suisun Bay in the preceding spring. The total number of delta smelt is not known, however, delta smelt are considered environmentally sensitive because they have a one year life cycle, unusually low fecundity for a fish with planktonic larvae, a limited diet, and reside primarily within the interface between salt and freshwater.

The threats to the population are multiple and synergistic. In decreasing order of importance, they include:

reductions in outflow from the Estuary,

entrainment to water diversions,

extremely high outflow,

changes in food organisms,

toxic substances,

disease, competition, and predation,

loss of genetic integrity by hybridization with the introduced wakasagi.

The Department in response to Fish and Game Commission directives initiated an intensive multidisciplinary study program in 1992 designed to monitor the population and to investigate all aspects of delta smelt biology in order to protect sufficient numbers to insure their long- term survival. Results from this study program are being used to make better informed water management decisions. The current research program is under the auspices of the Interagency Ecological Program and is funded by the California Department of Water Resources and U.S. Bureau of Reclamation.

By the end of 1993, the population trend for delta smelt changed from stable to increasing. However, annual changes in the population appear to be affected by the amount of outflow from the Estuary which varies from year to year due to precipitation and water management. In 1994, the population trend as measured by the fall abundance survey indicated that the population was at the lowest point in the 26 years of the survey. In 1995, with the extreme amount of rainfall we have recieved, there is concern that a large portion of larvae that were spawned may have been flushed out of the Estuary.