Anadromous, i.e. spends most of its time at sea but must return to freshwater to spawn. Reproduces in many rivers along the Atlantic coast of North America, from the St. Lawrence River to northern Florida. Has been successfully introduced to the Pacific coast in both the United States and Canada.
Prior to the early 2000s, only a single spawning ground was known in the St. Lawrence River, at Carillon (Ottawa River), but recent studies made it possible to locate a second spawning ground below a dam on the Des Prairies River, and there may be others.
Like salmon, American shad return to their birthplace, following olfactory clues, and stop feeding during the journey. They make their first migration around 4 years of age.
Spawning begins at dusk and continues throughout the night. Several males surround a female to encourage her to release her eggs. The fertilized eggs swell and sink to the bottom, where they settle. They hatch about ten days later, or earlier depending on the water temperature.
The fry and juveniles return to the sea during the summer. They are about 14 cm long by the time they reach the sea in late September.
The reproductive strategy of different American shad populations varies with latitude. Unlike shad in southern populations (in Florida and South Carolina), which reproduce only once in their lifetime, the St. Lawrence shad may return to breed for a number of consecutive years, in some cases up to 7 times. 86% of the shad that swim upstream in the St. Lawrence have done so at least once before. However, southern shad produce many more eggs (400,000 on average) than do St. Lawrence shad (130,000 on average). This strategy spreads the risks related to the northern climate over several years.