Threadfin Shad, Dorosoma petenense. Fish caught from shore at San Carlos Nuevo Guaymas, Sonora, August 2014. Length: 25 cm (9.8 inches). Photograph courtesy of by Isaias Ruiz, Guaymas, Sonora.

The Threadfin Shad, Dorosoma petenense, is a member of the Herring or Clupeidia Family, and is known in Mexico as sardina maya. There are five global members of the Dorosoma genus, of which two are found in Mexican waters, one in the Atlantic and this species which is found in both the Atlantic and the Pacific Oceans.

The Threadfin Shad has a deep compressed body with a depth that is 28% to 32% of standard length. They are gray-blue dorsally and silvery on their sides with a dark spot just behind their gill covers. Their fins are yellowish.  Their head has a pointed snout with a small mouth that opens at the front. Their anal fin has 17 to 27 rays and is short and originates behind the dorsal fin; their caudal fin is deeply forked; their dorsal fin has 11 to 15 rays and last dorsal ray is elongated; and, their pelvic fins have 1 spine and 7 rays and are low on the body and below the dorsal fin. Their body is covered with large scales.

The Threadfin Shad are a small pelagic coastal schooling species that form very large schools in rivers, large streams, and reservoirs in both marine and brackish shallow waters, at depths up to 10 m (35 feet). They reach a maximum of 33 cm (13 inches) in length and are virtually weightless. They have a tendency to collect on the water surface at dawn and at dusk. They feed on detritus, phytoplankton, and zooplankton. They are a favorite prey of catfish, largemouth bass, smallmouth bass, and striped bass. Reproduction occurs via dioecism with each female releasing 2,000 to 24,000 eggs per year that adhere to submerged and floating objects. They are very sensitive to changes in water conditions and are subject to large die-offs if water temperatures drop below 6^oC (42^oF). They are native to the southeast United States but have been introduced to several areas, including rivers in California and Oregon that drain into the Pacific, primarily as a forage fish for larger fish. They have lifespans between two and three years. The Threadfin Shad 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 Threadfin Shad is a resident of Mexican waters of both the Atlantic and the Pacific Oceans. In the Atlantic they are found in all coastal rivers that drain into the Gulf of Mexico and the Caribbean; in the Pacific they are only found in the rivers of the extreme northeastern section of Sea of Cortez.

In waters of the Atlantic the Threadfin Shad can be confused with the Atlantic Thread Herring, Opisthonema oglinum (caudal fin with black tips) and the Gizzard Shad, Dorosoma cepedianum (large bulbous snout; very deep body). In waters of the Pacific the Threadfin Shad is similar to the Deepbody Thread Herring, Opisthonema libertate, the Middling Thread Herring, Opisthonema medirastre, and the Slender Thread Herring, Opisthonema bulleri, but all lack the black spot on their gill covers.

From a conservation perspective the Threadfin Shad is currently considered to be of Least Concern with stable, widely distributed populations. They have a limited interest to humans other than utilization as a live bait.  They date to the Plio-Pleistocene Period, 5,000,000 years ago, with fossils collected from the Pecos River Valley in State of New Mexico.