There are two types of shad - the twaite and allis. The above example is an allis shad as twaite's seldom grow larger than 2lbs.
British Sea Fishing tells us that The only other differences are that allis shad have more than seventy scales along the lateral line and more than ninety gill rakers, whereas twaite shad have fewer than seventy scales along the lateral line and fewer gill rakers. The dark spots on the flanks and tail vary within and between species meaning they cannot be used for identification purposes. The two species can also interbreed, meaning that some shad exist which have numbers of lateral line scales and gill rakers between the figures mentioned above.
A rare fish to see on the market as they are very much an occasional by-catch and not landed in any quantity.