Middling Thread Herring, Opisthonema medirastre

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, California.

Middling Thread Herring, Opisthonema medirastre. Gill raker count on lower arch: 70.

Middling Thread Herring, Opisthonema medirastre, Juvenile. Fish provided by the commercial bait salesmen of Puerto Los Cabos, Baja California Sur, September 2015. Length: 8.7 cm (3.4 inches).

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 photograph courtesy of Chris Wheaton, Fullerton, California.

Middling Thread Herring, Opisthonema medirastre. Fish provided by the commercial bait salesmen of Puerto Los Cabos, Baja California Sur, June 2018. Length: 27 cm (11 inches).

Middling Thread Herring, Opisthonema medirastre. Fish provided by the commercial bait salesmen of Puerto Los Cabos, Baja California Sur, May 2022. Length: 30 cm (12 inches). Gill raker count on lower arch: 70. This fish has a length that is equal to the maximum known length for this species.

Middling Thread Herring, Opisthonema medirastre. Fish caught in coastal waters off Loreto, Baja California Sur, June 2024.  Length: 30 cm (12 inches).

The Middling Thread Herring, Opisthonema medirastre, is a member of the Herring or Clupeidia Family, and is known in Mexico as sardina crinudo machete or simply grande sardina. There are five global members of the genus Opisthonema, of which four are found in Mexican waters, one in the Atlantic and three in the Pacific Ocean.

The Middling Thread Herring has a moderately deep compressed body with a depth that is 32% 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 a pair of black spots on top of their back adjacent to the origin of the dorsal fin (as pictured above). 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 has a short base that is shorter than the head and located well behind the dorsal fin with 13 to 21 rays; their dorsal fin is located mid-body and has an elongated filamentous ray at its end that does not reach the caudal fin base and has 12 to 23 rays; and their pectoral fins are short and do not reach the dorsal fin origin. They have 41 to 69 lower gill rakers in fish that are greater than 14 cm (5.5 inches) in length (a key to identification). Their body is covered with small scales.

The Middling Thread Herring is small pelagic coastal schooling species found at depths up to 10 m (33 feet). They reach a maximum length of 30 cm (12 inches) and are virtually weightless. The Middling Thread Herring is poorly studied with very limited information available about their lifestyle and behavioral patterns including specific details on age, growth, longevity, movement patterns, diet, habitat use, and reproduction.

The Middling Thread Herring is a resident of all Mexican waters of the Pacific Ocean 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 36% to 40% of standard length; pectoral fin reach dorsal origin; black spots on sides; 63 to 110 gill rakers in adults), the Slender Thread Herring, Opisthonema bulleri (body depth 30% 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).

From a conservation perspective the Middling Thread Herring is currently considered to be of Least Concern with stable, widely distributed populations. They are collected by deep-water purse seines mixed in with the Deepbody Thread Herring, Opisthonema libertate at levels of 50,000 to 1000,000 tons per year.  They are sold commercially for human food consumption frozen through the majority of the major Mexican food markets throughout the country, and also  used to make fish meal and fish oil.