 Longfin Smelt (Spirinchus thaleichthys)
 Distribution of longfin smelt in San
Francisco Bay
Longfin smelt are pelagic, estuarine fish which range from Monterey Bay
northward to Hinchinbrook Island, Prince William Sound Alaska. In California, they have been commonly
collected from San Francisco Bay, Eel River, Humboldt Bay and Klamath River. Presently, the only
California collections made in the 1990s have been from the Klamath River and San Francisco Bay.
Longfin smelt reach a maximum size of about 150 mm TL. Longfin smelt comprise a
small portion of the "whitebait" fishery in San Francisco Bay and have no sport fishery value.
Maturity is reached toward the end of their second year. As they mature in the fall, adults
found throughout San Francisco Bay migrate to brackish or freshwater in Suisun Bay, Montezuma Slough,
and the lower reaches of the Sacramento and San Joaquin Rivers. Spawning probably takes
place in freshwater.
In April and May, juveniles are believed to migrate downstream to San Pablo Bay; juvenile longfin smelt
are collected throughout the Bay during the late spring, summer and fall , and occasionally venture into
the Gulf of the Farallons. Juveniles tend to inhabit the middle and lower portions of the water column.
A relationship between freshwater outflow and longfin smelt abundance has been identified. The
overall effect of high freshwater outflow appears to be an increase in the
amount and quality of nursery habitat (i.e. creation of brackish water habitat in San Pablo Bay) and a
broader dispersal of YOY fish, potentially increasing feeding opportunities and reducing density-dependent
mortality. Differences in overall dispersal of YOY longfin smelt are
not readily detectable with the data presented, yet increased use of San Pablo Bay was noted in both high
abundance years. Alternately, low freshwater outflows during the winters of 1987-1992 are believed to be
the main reason for the decline in longfin smelt during the same period.
An Annual Abundance Index for the
juveniles is generated by the San Francisco Bay Monitoring Program
using midwater trawl data.
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