Middling thread herring (Opisthonema medirastre) is found from Los Angeles Bay in southern California, USA and the Gulf of California, Mexico to Sechura Bay in northern Peru (SAGARPA, 2012). According to Bakun et al. (2009), the Gulf of California is intermittently subject to important influxes and major exchanges of small pelagic forage fishes, such as Opisthonema medirastre, both northwards and southwards the mouth of the gulf. Although the stock structure in the Pacific Ocean is not known, recent research suggests that O. medirastre in Northwestern Mexican Pacific waters do not belong to different populations or stocks.

In the Sonora region, this species is captured in a very low proportion (as compared to the dominant species, the Pacific thread herring). In the southern Gulf, in Sinaloa and Nayarit, the proportion of the three Ophistonema species is more similar and assessment is conducted separately (Morgan et al. 2016)​. Therefore, an assessment unit is considered in Sinaloa and Nayarit for middling thread herring.