[Karl F]

Dorsal spines (total): 0-0; Dorsal softrays (total): 15-20; Anal spines: 0-0; Anal soft-rays: 15-21; Vertebrae: 47-53. Moderately compressed, belly with distinct keel of scutes. Upper jaw with a distinct notch; lower jaw rising steeply within mouth; minute teeth present at front of jaws (disappearing with age). Lower gill rakers 41 to 52 (fewer in fishes under 10 cm standard length), slender. Back dark blue, sometimes bluish-grey; a dark spot on shoulder (Ref. 188). Peritoneum black (Ref. 7251). Branchiostegal rays 7 (Ref. 4639).
The blueback herring and alewife are of similar shape and general appearance, and distinguishing between them is difficult. Bluebacks tend to have a smaller eye than alewives, with the eye diameter usually smaller than the snout length. As their name implies, these fish often have dark blue backs. Both alewife and blueback herring are silvery in color and have a series of scutes (modified scales that are spiny and keeled) along their belly; however, the dorsal area of alewife are bronze in color whereas blueback herring are deep bluish green. Alewife are more strongly compressed, deep, their body is less elongated and they have a much larger eye than blueback herring. However, the most distinguishing characteristic of these species is the color of their peritoneum or the lining of the abdominal cavity. An alewife's peritoneum is pale with dusky spots, whereas a blueback herring's is black to dusky in color. Alewife and blueback herring are so difficult to distinguish from each other, in most instances, they are collectively termed "river herring".
Max size:40.0 cm SL (male/unsexed); max.weight: 200.0 g; max. reported age: 8 years.