Alosa fallax is a member of the herring family. It is a planktivorous fish with silvery white sides and a deep blue colouration on its back. It is most commonly recorded at a size between 20-40 cm but has been reported to reach 60 cm. Like other shads its upper jaw is distinctly notched in the mid line and the gill cover has distinct radiating ridges.The twaite shad is distinguished from the other European shad, the allis shad (Alosa alosa), by having between 40-60 gill rakers on the first gill arch while Alosa alosa has between 80-130 (Whitehead, 1985). The twaite shad is anadromous in that it spends its life in the ocean but enters rivers in April and May to spawn before returning to the sea. Juveniles remain in the rivers for up to 24 months (Muus & Nielson, 1999).