American Shad

American Shad, Alosa sapidissima

The American Shad, Alosa sapidissima, whose common Spanish name is sábalo americano, is a member of the Herring or Clupeidae Family, known collectively as sardinas in Mexico. Globally, there are 23 species in the genus Alosa and this species is the only one found in Mexican waters.

The American Shads have moderately deep, fusiform, and compressed bodies with a depth that is 27% to 31% of standard length. They are silvery with a blue-green metallic sheen dorsally and transition to white ventrally with one or more dark spots adjacent to the top of their gill covers. Their head has small eyes and an oblique mouth that opens at the front. Their anal fin is short with 18 to 24 rays and is located well behind the dorsal fin; their caudal fin is forked; their dorsal fin has 15 or 19 rays and is located before the center of the body. They have over 60 gill rakers, which are long and slender. Their belly has a distinct keel.

The American Shads are a small coastal pelagic species found on the surface during summer months and at depths up to 820 feet at other times of the year. They reach a maximum length of 76 cm (30 inches) and weight of 5.5 kg (11 pounds); they are dimorphic with females being as much as three times larger than males. They are widely distributed, common, and locally abundant forming large schools. They are highly migratory and are believed to travel over 19,000 km (12,000 miles) during their lifetime. They mostly consume plankton (copepods and mysids) and occasionally feed on small fish; they are believed to help control the populations of the species they consume. They have the ability to detect sound, which helps them avoid predation, however, they are important forage fish for large predatory fish, dolphins, bears, birds, and humans. They make annual migrations of up to 630 km (400 miles) to freshwater rivers for reproduction. Populations vary from iteroparous, making several spawning trips to freshwater, to semelparous, making one trip for spawning and then dying. Reproduction is oviparous with each female releasing eggs in batches of 30,000 that are fertilized externally, with total annual egg production of 200,000 to 600,000 per female. Adults return fairly quickly to the ocean after spawning and virtually disappear moving to deeper waters in the winter. Eggs are pelagic and can move independently for several miles downstream where they hatch within 10 days. Young collect in large schools and return to the ocean in autumn arriving as 3.8 cm (1.5 inch) to 11.4 cm (4.5 inch) miniature adults. They have a lifespan of up to 13 years.

In Mexican waters the American Shads have a limited distribution being found in the coastal waters of northwest Baja. This species is native to the east coast of the United States and was introduced to the Sacramento River in the 1870s and subsequently spread both northward and southward.

The American Shad is most likely confused with the Alewife, Alosa pseudoharengus (steeply rising lower jaw; large eyes; less than 60 gill rakers).

The American Shads are considered to have both commercial and recreational value. They are caught by hook and line on spinning gear and fly fishing tackle, by cast nets during migration runs, and as a by-catch of menhaden and shrimp trawlers. They have been a major food fish of Native Americans for centuries, however, they are prone to infection by a wide variety of nematodes. Shad roe is considered a delicacy. From a conservation perspective they are currently listed as of Least Concern, being common over a wide range with stable populations estimated to be in excess of 1 million individuals. However, their populations have been subject to significant declines due to overfishing and habitat destruction (dams). They are being bred in fish hatcheries in a variety of locations.

American Shad, Alosa sapidissima. Fish caught in the Cooper River, South Carolina, March 2011. Length: 51 cm (20 inches). Catch, photo, and identification courtesy of Josh Leisen (lifelistfishing.com), Gaylord, MI.