The American shad or Atlantic shad, Alosa sapidissima[1], is a species of anadromous fish in family Clupeidae of order Clupeiformes. It is not closely related to the other North American shads. Rather, it seems to form a lineage that diverged from a common ancestor of the European taxa before these diversified (Faria et al. 2006).

The shad spends most of its life at sea, but swims up fresh rivers to spawn. Unlike salmon, the fish survive breeding and can return to the sea; they do not inhabit fresh water except to spawn. At sea, shad are schooling fish; thousands are often seen at the surface in spring, summer, and autumn. They are hard to find in the winter, as they tend to go deeper before spawning season; they have been pulled up in nets as deep as 65 fathoms. Like other herrings, the American Shad is primarily a plankton feeder, but will eat small shrimp and fish eggs. Occasionally they eat small fish, but these are only a minor item in their general diet. The sexually mature fish enter the streams in spring or early summer when the river water has warmed to 50° to 55°F. Cooler water appears to interrupt the spawn. Consequently the shad run correspondingly later in the year passing from south to north along the coast, commencing in Georgia in January; in March in the waters tributary to Pamlico and Albemarle Sounds; in April in the Potomac; and in May and June in northern streams generally from Delaware to Canada. In large rivers they run far upstream especially in the open rivers of the southeast. The apparent longest distance is in the St. Johns River of Florida, an extremely slow (1" drop per mile) river that widens into large lakes; shad have been found 375 miles upriver. The fish select sandy or pebbly shallows for spawning grounds, and deposit their eggs mostly between sundown and midnight. Females release eggs in batches and produce about 30,000 eggs per batch, though as many as 156,000 have been estimated in very large fish. Total annual egg production is 200,000 - 600,000 eggs per female with larger fish producing more. In rivers north of Cape Fear the spent fish, now very emaciated, begin their return journey to the sea immediately after spawning. In southern rivers, most shad die after spawning. The eggs are transparent, pale pink or amber, and being semi-buoyant and not sticky like those of other river herrings, they roll about on the bottom with the current. The eggs hatch in 12 to 15 days at 52° (12°C), in 6 to 8 days at 63° (17°C), which covers the range characteristic of Maine and Bay of Fundy rivers during the season of incubation. The larvae are about 9 to 10 mm. long. The young shad remain in the rivers until fall, when they move down to salt water; they are now 1.5 to 4.5 inches long, resembling their parents in appearance.