Delta smelt

Delta smelt (Hypomesus transpacificus) is an endangered slender-bodied smelt, about 5 to 7 cm (2.0 to 2.8 in) long, in the Osmeridae family. Endemic to the upper Sacramento-San Joaquin Estuary of California, it mainly inhabits the freshwater-saltwater mixing zone of the estuary, except during its spawning season, when it migrates upstream to freshwater following winter "first flush" flow events (around March to May). It functions as an indicator species for the overall health of the Delta's ecosystem.

Because of its one-year lifecycle and relatively low fecundity, it is very susceptible to changes in the environmental conditions of its native habitat. Efforts to protect the endangered fish from further decline have focused on limiting or modifying the large-scale pumping activities of state and federal water projects at the southern end of the estuary. However, these efforts have not prevented the species from becoming functionally extinct in the wild.

Taxonomy and evolution

The delta smelt is one of five currently recognized species within the Hypomesus genus, which is part of the larger Osmeridae family of smelts. The genus has been subject to many revisions since it was first classified by Gill in 1863. The first major revision occurred in 1963, when the Osmeridae family was reexamined by Canadian ichthyologist Donald Evan McAllister. Expanding on Japanese researcher Hamada's earlier determination that H. olidus was not a monolithic widespread species, but rather one of three distinct species of Hypomesus, McAllister assigned them new names, and further delineated what he believed were four subspecies. This was the first description of H. transpacificus; named for its supposed occurrence on both sides of the Pacific, and also "to the friendship of Japanese and Canadian ichthyologists." He separated these geographically isolated populations into separate subspecies: H. t. transpacificus and H. t. nipponensis.