Flowing water carry the larvae downstream toward saltier water. Juvenile alewife and blueback herring migrate from the Bay to coastal waters in the early fall and remain there until reaching sexual maturity in three to six years. Larger than these river herring are the shad. Prompted by rising temperatures they, too, leave the ocean and return to the waters where they were born. Both species generally spawn from March to June.
Males arrive on the spawning grounds first, followed by egg-laden females.
A female releases 100,000 to 600,000 eggs into the water to be fertilized by several males. Adult shad return to the ocean soon after spawning. The transparent fertilized eggs are carried along by the current. The larvae hatch in four to 12 days. Juvenile shad spend their first summer in fresh water.