Threadfin Shad, Dorosoma petenense
The Threadfin Shad, Dorosoma petenense, whose common Spanish name is sardina maya, is a member of the Herring or Clupeidia Family, known collectively as sardinas in Mexico. There are only five global members of the Dorosoma genus, two of which are found in Mexican waters, one in the Atlantic and one in both the Atlantic and the Pacific.
The Threadfin Shads have deep compressed bodies 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 is short with 17 to 27 rays and originates behind the dorsal fin. Their caudal fin is deeply forked; their last dorsal ray is elongated; and their pelvic fins are low on the body and below the dorsal fin. Their body is covered with large scales.
The Threadfin Shads 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 ten meters (35 feet). They reach a maximum length of 33 cm (13 inches) 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. They have a lifespan between two and three years. 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 6oC (42oF). 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.
In Mexican waters the Threadfin Shad are found in both the Atlantic and the Pacific. In the Atlantic they are found in all coastal rivers; in the Pacific they are only found in the rivers of the extreme northern 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.
The Threadfin Shads date to the Plio-Pleistocene Period (5 million years ago) with fossils collected from the Pecos River Valley in New Mexico.