The alewife reaches an average length of four to nine inches and has a life span of about six to eight years. Its large mouth opens to below the front of its eyes, and its lower jaw projects forward. The body is deep with a row of sharp scales on the midline of the belly. The fins are small, and the dorsal fin does not have a long projection. Only one dorsal fin is present. The back is blue or blue-green. The sides are silver, and the belly is white. A small dark spot is present in back of each gill cover. There is no lateral line. Cycloid scales cover the body, but scales are absent on the head. The adipose eyelid is easily seen.
This species travels in schools. It is active at all depths in the water. Males mature in one to two years, while females mature in two to three years. Spawning occurs in summer at night in shallow water. The alewife eats plankton, algae and small aquatic invertebrates. Populations are not stable. Sometimes huge numbers of alewives die and wash to shore. This species invaded the Great Lakes through the Welland Canal from the 1930s through 1960s. Chinook and coho salmon were then introduced to the Great Lakes intentionally to feed on this species. It is preyed upon by many other fish species as well.