The most reliable identifying characteristic is the color of the tissue lining the body cavity, dark brown or blackish in the blueback herring and grey or silvery in the alewife. The alewife occurs from Newfoundland to South Carolina, while the blueback herring occurs from Nova Scotia to the St. Johns River in Florida. The onset of spring spawning is related to temperature, and thus varies with latitude. Alewife spawn in rivers and tributaries from northeastern Newfoundland to South Carolina, but are most abundant in the mid-Atlantic and northeastern states. Blueback herring spawn from Nova Scotia to northern Florida, but are most numerous in warmer waters from Chesapeake Bay south.
Alewife usually spawn three to four weeks earlier than blueback herring in the same watershed.
Blueback herring prefer to spawn in swift flowing sections of freshwater tributaries, channel sections of fresh and brackish tidal rivers, and coastal ponds, over gravel and clean sand substrates, especially in northeastern rivers where alewife and blueback herring coexist. Mature river herring broadcast their eggs and sperm simultaneously into the water column and over the substrate. Larvae begin to feed externally three to five days after hatching, and transform gradually into the juvenile stage. Juveniles remain in freshwater nursery areas in spring and early summer, feeding mainly on zooplankton. As water temperatures decline in the fall, juveniles move downstream to more saline waters and eventually to the sea.