Middling Thread Herring, Opisthonema medirastre
The Middling Thread Herring, Opisthonema medirastre, whose common Spanish name is sardina crinudo machete, is a member of the Herring or Clupeidia Family, known collectively as sardinas in Mexico. There are only five global members of the genus Opisthonema, four of which are found in Mexican waters, one in the Atlantic and three in the Pacific.
The Middling Thread Herrings have moderately deep compressed bodies with a depth that is 30 to 36% of standard length; adults have deeper bodies than juveniles. They are gray-green dorsally and silvery on their sides with a yellow stripe mid-body. They have a black spot just behind their gill covers and one or two black spots on top of their back adjacent to the dorsal fin. The inner portions of their anal and dorsal fins are pale yellow and the outer halves are clear. Their caudal fin tips are dusky. Their head has a short oblique mouth that opens at the front. Their anal fin base is shorter than the head and located well behind the dorsal fin. Their dorsal fin is located mid-body and has an elongated filamentous ray at its end that does not reach the caudal fin base. They have short pectoral fins that do not reach the dorsal fin origin and 41 to 69 lower gill rakers (a key to identification). Their body is covered with small scales.
The Middling Thread Herrings are a small pelagic coastal schooling species found at depths up to 10 meters (35 feet). They reach a maximum length of 30 cm (12 inches) and are virtually weightless. They are a poorly studied species and very little is known about their behavioral patterns.
In Mexican waters the Middling Thread Herrings are found in all waters of the Pacific with the exception that they are absent from the northern 20% of the Sea of Cortez.
The Middling Thread Herring can be easily confused with the Deepbody Thread Herring, Opisthonema libertate (body depth 38% of standard length; pectoral fin reach dorsal origin; black spots on sides), the Slender Thread Herring, Opisthonema bulleri (body depth 31 to 32% of standard length; 25 to 36 gill rakers), and the Threadfin Shad, Dorosoma petenense (long anal tail base; ventral profile with strongly convex keel).
The Middling Thread Herrings are too rare to be of interest to most. They are collected by deep-water sardine trawlers and sold commercially through the majority of the major Mexican food markets country wide.
Middling Thread Herring, Opisthonema medirastre, juvenile. Fish provided by the commercial bait salesmen of Puerto Los Cabos, Baja California Sur, August 2007. Length: 12 cm (4.7 inches). Note: the body depth of this juvenile is 31% resulting in a difficult identification. Identification courtesy of H.J. Walker, Jr., Scripps Institution of Oceanography, La Jolla, CA.
Middling Thread Herring, Opisthonema medirastre, juvenile. Fish caught in coastal waters of Bahia de Los Angeles, Baja California, July 2015. Length: 13.0 cm (5.1 inches). Catch and photo courtesy of Chris Wheaton, Loreto, Baja California Sur.