The first step to understanding the ecological needs of a threatened or endangered species is to identify its geographic range and its temporal distribution within that range. Longfin smelt, listed under the California Endangered Species Act as threatened, is distributed throughout coastal and estuarine areas of the northern Pacific Ocean. The San Francisco Estuary is close to the southern end of its range. The distribution of the fish’s life stages is poorly known, in part because targeted searches for longfin smelt have been few and the species is not sampled well by trawl surveys that have been carried out in the Bay-Delta for decades; also because a large percentage of the population spends a large portion of its life cycle in the ocean where no surveys at all occur. Despite the inadequacies of the existing surveys to monitor longfin smelt, it is prudent to review the historical survey data for preliminary insights into the distribution of the fish. Using available sampling data from 1959 to 2012 from 1,203 monitoring stations, 1,035,183 longfin smelt (LFS) were observed at 643 stations (53%) in an area from Central San Francisco Bay (Tiburon) in the west, to Colusa on the Sacramento (Sacramento Valley region) in the north, Lathrop on the San Joaquin River (border of South Delta and San Joaquin River regions) to the east and South San Francisco Bay (Dumbarton Bridge) to the south, an area of approximately 137,500 ha. Juvenile longfin smelt were observed between the western bays and Suisun Marsh. During the spawning periods, adults were recorded from those locations, the upper Delta reaches, and in San Francisco Bay, indicating that LFS spawning habitat may extend further upstream and downstream than presumptive longfin smelt rearing habitat. Longfin smelt appeared to migrate completely out of the lower rivers by July, but some adults remained in downstream areas of the estuary areas, suggesting not all individuals migrate out beyond the Golden Gate. Observations of longfin smelt increased throughout fall months across all areas, suggesting immigration and emigration through the estuary. This data review provides managers and scientists an improved, but still limited, record of the spatial and temporal extent of the range of longfin smelt within the Estuary.
Merz JE, Bergman PS, Melgo JF, Hamilton S. 2013. Longfin smelt: Spatial dynamics and ontogeny in the San Francisco Estuary, California. California Fish and Game 99:122-148.