ESWR page - Endangered Species & Wetlands Report

I. NATURAL HISTORY AND STATUS OF THE DELTA SMELT
A. NATURAL HISTORY
1. Description
The delta smelt (Hypomesus transpacificus) is a small (50-80 mm in length for most
adults), nearly translucent, steely-blue, osmerid fish endemic to the San Francisco
Bay/Sacramento-San Joaquin Delta Estuary, in California (Delta). The delta smelt was
described as follows by Moyle (2002):
…slender-bodied fish that typically reach 60-70 mm SL (standard length;
2.36—2.76 in), although a few may attain 120 mm (4.73 in) SL. The
mouth is small, with a maxilla that does not extend past the mid-point of
the eye. The eyes are relatively large; the orbit width contained
approximately 3.5-4 times within head length. Small pointed teeth are
present on the upper and lower jaws. The first gill arch has 27-33 gill
rakers and there are 7 branchiostegal rays. The gill covers lack strong
concentric striations. The pectoral fins reach less than two-thirds of the
way to the bases of the pelvic fins. There are 8-11 (usually 9-10) dorsal fin
rays, 8 pelvic fin rays, 10-12 pectoral fin rays, and 15-19 anal fin rays. The
lateral line is incomplete and has 53-60 scales along it. There are 4-5
pyloric caeca. Live fish are nearly translucent and have a steely-blue sheen
to their sides. Occasionally there maybe one chromatophore (a small dark
spot) between the mandibles, but usually there is none.
2. Taxonomy
Delta smelt is one of six species currently recognized in the Hypomesus genus
(Saruwatari et al. 1997; as cited in Bennett 2005) and genetic analyses have confirmed
that it is a well-defined species with a single intermixing population (Stanley et al. 1995;
Trenham et al. 1998). Within the genus, delta smelt is most closely related to surf smelt
(H. pretiosis), a species common along the western coast of North America. In contrast,
delta smelt is a comparatively distant relation to the wakasagi (H. nipponensis), which
was introduced into Central Valley reservoirs in 1959 and is now sympatric with delta
smelt in the estuary. Delta smelt and wakasagi can also be distinguished using
morphometric and pigmentation characteristics (Moyle 2002).
The U.S. Fish and Wildlife Service (USFWS) has repeatedly concluded the delta smelt is
a distinct species: in an October 1991 proposed listing (56 FR 50075 50084), a March
1993 determination of threatened status (58 FR 12854 12864), a January 1994 revised
critical habitat determination (59 FR 852 862), a December 1994 critical habitat
determination for the species, and a 2004 5-Year Status Review for the species (USFWS
2004b).
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