Middling thread herring

Opisthonema medirastre is a small fish of the family Dorosomatidae that schools near the coast in the eastern Pacific ^[1],[2]. Its known southern limit is Sechura Bay, in northern Peru ^[2]. The IUCN classifies it as Least Concern ^[3].

It reaches 30 cm in total length ^[2]. To tell it from its relatives, biologists count the gill rakers on the ceratobranchial of the first arch: fish longer than 14 cm have between 41 and 69 ^[2]. Berry and Barrett were the first to propose this method, in 1963, to separate Opisthonema bulleri, Opisthonema medirastre and Opisthonema libertate ^[4].

Counting is not always enough. Gill-raker characteristics overlap among the species, which leads to identification errors, and for that reason Mexican fishery statistics record Opisthonema spp. without separating them ^[4]. In the southern Gulf of California, a genetic technique (PCR-RFLP) based on the mitochondrial COI gene told the 3 species apart unambiguously; 43 sequences of that gene were reviewed, 16 of them from Opisthonema medirastre ^[4].

Similar species

• Opisthonema libertate: It shares the eastern Pacific with it and is hard to tell apart by morphology: gill rakers must be counted or a genetic analysis used ^[4].
• Opisthonema bulleri: It is the third species of the group in the eastern Pacific and is separated the same way, by gill rakers or by genetics ^[4].

It lives in the eastern Pacific, from Los Angeles (California) to Sechura Bay, in Peru ^[2]. It is a marine, pelagic, neritic species found between 0 and 50 m depth, usually close to shore in schools ^[2]. Another study places the southern limit of the 3 Mexican Pacific Opisthonema species in northern Peru ^[4].

It also occurs in the Galápagos Islands (Ecuador): there, 160 juveniles were caught off southern Santa Cruz, in the warm, wet season of a year-long sampling effort ^[5].

Here we summarise one study of juveniles in the Galápagos. The 160 juveniles measured between 16 and 48 mm standard length and were between 30 and 88 days old ^[5]. They had hatched at the start of the warm, wet season and were growing at 0.4 mm per day ^[5].

According to that study, spawning in the archipelago takes place in the warm, wet season, mostly when ENSO is in a neutral phase ^[5]. As an adult it feeds on crustaceans and pteropods ^[2].

The IUCN places Opisthonema medirastre in the global category Least Concern (LC), according to the 2019 assessment ^[3]. FishBase records the same category dated 14 November 2019 ^[2].

It supports a minor commercial fishery and is sometimes used as bait ^[2]. In Mexico, the 3 Opisthonema species are a resource of great commercial importance in the Pacific, but landings are recorded together as Opisthonema spp. ^[4].