During fall, the water begins to cool and the young move downstream, beginning their first seaward migration. The herring's feeding habits vary with their age and habitat. While in fresh water, alewife feed mainly on single-cell algae and microscopic invertebrates. When they move into saltier water, the diet shifts to small shrimp and fish. Blueback feed primarily on plankton, which are tiny floating organisms.
Herring are food for estuarine and coastal predators, including striped bass.
Eggs, juveniles, and adults are all important forage for many freshwater and marine fishes, birds, amphibians, reptiles, and mammals. Historically, the ample supply of river herring supported a large fishery. Colonists stored salted herring for winter food and used the oily backbones of blueback herring as lamp wicks. Today, herring are harvested using pound, gill, and fyke nets and haul seines. Some herring are sold for consumption, and can be eaten fresh, pickled, or smoked.