Alosa alosa is a herring-like, planktivorous fish with silvery white sides and a deep blue colouration on its back. It is most commonly recorded at a size between 30 -50 cm but has been reported to reach 70 cm. Like other shads its upper jaw is distinctly notched in the mid line and the gill cover has distinct radiating ridges.The allis shad is distinguished from the other European shad, the twaite shad (Alosa fallax), by having between 80-130 gill rakers on the first gill arch while Alosa fallax has between 40-60. The allis 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.