Fish Identification Photos: Middling Thread Herring, Opisthonema medirastre: The Middling Thread Herring has a wide body which has a depth that is 36 to 38 percent of its length. It is gray-green on its back with silver white sides.

The Middling Thread Herring has a black spot on the top of its gill cover with a yellow strip that follows along its upper flank. The inner half of the caudal and dorsal fins are pale yellow, the outer halves clear, and the tip of the caudal fin is dusky.

The Middling Thread Herring has a long filamentous ray originating from the last dorsal ray that does not reach the caudal fin. It has short pectoral fins that do not reach the origin of the dorsal fin, and a anal fin with a short base that originates well behind the dorsal fin.

A key to identification of the Middling Thread Herring is a gill raker count of 41 to 69. The Middling Thread Herring reaches a length of 11 inches and is virtually weightless. It is normally found in the first 30 feet of the water column in small schools over sandy bottoms.

The Middling Thread Herring can be easily confused with the other three Thread Herrings found in Mexican waters: the Pacific Thread Herring or Deepbody Thread Herring, Opisthonema libertate (body depth 38 percent of body length, pectoral fin reaches dorsal origin, black spots on sides), the Slender Thread Herring, Opisthonema bulleri (body depth 31 to 32 percent of body length, 25 to 36 gill rakers), and the Threadfin Shad, Dorosoma peteneuse (long anal tail base and a ventral profile with a strongly convex keel).

The Middling Thread Herring is a coastal pelagic species that is reported to be found in all Mexican waters with the exception of the northern 25 percent of the Sea of Cortez.

The Middling Thread Herring is a member of the Clupeida family which consists of Herrings, Menhadens, Pilchards, Piquitingas, Sardines, Shads and Threadfin herrings which are known in Mexico as sardinas. The Genus Opisthonema, the Thread Herrings, have deep bodies with an oblique short mouth, a long filamentous ray originating from the last dorsal fin ray, and a short anal fin base that originates well behind the dorsal fin. Globally, 227 species of Culpeida have been identified of which only 11 are found in the tropical Eastern Pacific. For the Thread Herring, only five are known globally of which four reside in Mexican waters.

Middling Thread Herring, Opisthonema medirastre: Juvenile. A by-catch when one purchases a “scoop of bait.” Caught in the surf zone, La Playita, San Jose del Cabo, Baja California Sur, Mexico, in August 2007, early morning in 5 to 10-foot deep water, utilizing a cast net by panguero bait salesmen. Size 5 inches. Note: the body depth of this juvenile is 31 percent, affording a difficult identification. Identification courtesy of H.J. Walker, Jr., Scripps Institute of Oceanography, La Jolla, Calif. Description and photo courtesy of John Snow.