Slender Thread Herring, Opisthonema bulleri
Slender Thread Herring, Opisthonema bulleri, Juvenile. Fish provided by the commercial bait salesmen of Puerto Los Cabos, Baja California Sur, December 2020. Length: 10.5 cm (4.1 inches). Note: the body depth of this juvenile is 27.1%. My gill raker count for this fish was 29 on the lower arch. I have a photo posted of a 8.7 cm Juvenile Middling Thread Herring that has a body depth of 29.4% obtained from the same location posted on this www site. This might be a Middling Thread Herring. Complicating the identification is that the Slender Thread Herring is not known to be a resident of the southeast coast of the Baja.
Slender Thread Herring, Opisthonema bulleri, Juvenile. Fish provided by the commercial bait salesmen of Puerto Los Cabos, Baja California Sur, December 2020. Length: 10.5 cm (4.1 inches). Note: the body depth of this juvenile is 27.1%. This might be a Middling Thread Herring.
Slender Thread Herring, Opisthonema bulleri. Fish caught from coastal waters off Puerto Peñasco, Sonora, April 2021. Length: 24 cm (9.4 inches). Catch, photograph and identification courtesy of Chris Moore, Peoria, Arizona.
The Slender Thread Herring, Opisthonema bulleri, is a member of the Herring or Clupeidia Family, and is known in Mexico as sardina crinuda azul. 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 Slender Thread Herring has a relatively slender body with a depth that is 30% to 32% of standard length. They are gray-green dorsally and silvery on their sides with a yellow stripe mid-body and one or two spots on top of the back under the dorsal fin. They have a black spot just behind their gill covers. The inner portions of their anal and dorsal fins are pale yellow and the outer halves are clear. Their caudal fin has dusky tips. Their head has a short oblique mouth that opens at the front and is equipped with small conical teeth. 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; their pectoral is are short and do not reach the dorsal fin origin; and, their pelvic fins have 7 rays. Adult fish that are greater than 14 cm in length have 25 to 36 lower gill rakers (a key to identification). Their body is covered with small scales.
The Slender Thread Herring is small pelagic coastal schooling species found at depths up to 50 m (165 feet). They reach a maximum length of 24 cm (9.4 inches) and are virtually weightless. They consume crustaceans and pteropods. The Slender 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 Slender Thread Herring is a resident of all Mexican waters of the Pacific Ocean with the exception that they are absent from the coastal waters along both coasts of the Baja. If my identification is correct it would extend the known range of the Slender Thread Herring to the extreme southeast coast of Baja.
The Slender 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; 63 to 110 gill rakers in adults), the Middling Thread Herring, Opisthonema medirastre (body depth 32% to 36% of standard length; 41 to 69 gill rakers in adults), and the Threadfin Shad, Dorosoma petenense (long anal tail base; ventral profile with strongly convex keel).
From a conservation perspective the Slender 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 and Middling Thread Herring and sold commercially for human food consumption frozen but on a limited basis. They are also utilized to make fish meal and fish oil.