Allis Shad

The Allis Shad (Alosa alosa) is a large anadromous member of the herring family (Clupeidae), native to the eastern Atlantic from Scandinavia to Morocco and into the Mediterranean. It is the largest of the European shads, historically supporting significant seasonal river runs. Adults live in coastal marine waters, feeding on plankton, then migrate upstream in spring to spawn in fast-flowing gravel reaches of large rivers. Dam construction, river pollution, and habitat loss have caused severe population declines across its range, and the species is now considered threatened or extinct in many former river systems. It remains an indicator species for river connectivity and water quality, and conservation programs in France and Spain focus on restoring migratory access.

Allis Shad are identified by a deep, laterally compressed, silvery body with a strongly forked tail and a sawtooth keel of scutes along the belly.

• Single large dark spot just behind the gill cover, sometimes followed by a faint row of smaller spots
• Deeply notched upper jaw, giving a distinctive profile
• Large, easily shed cycloid scales
• Length typically 40-70 cm, with a laterally compressed, herring-like silhouette
• Distinguished from the similar Twaite Shad by having far more (60-100) close-set gill rakers and usually only one flank spot rather than a longer row

Allis Shad are anadromous, spending most of their adult life in coastal and shelf waters of the northeastern Atlantic, from Norway south to Morocco, with populations also entering the western Mediterranean. They tolerate a range of salinities and pass through brackish estuaries during their spring spawning migrations into large, fast-flowing rivers such as the Loire, Gironde, and Minho. Spawning occurs over clean gravel or sandy substrate in flowing water, and juveniles rear in freshwater and estuarine nursery areas before returning to sea in autumn. Because their life cycle depends on both open ocean and unobstructed river access, dams and weirs have eliminated the species from many historic river systems.

Allis Shad form large schools while at sea, feeding mainly on planktonic crustaceans and small fish by filtering water through numerous fine gill rakers. In spring, mature adults undertake synchronized upstream spawning migrations, gathering in river reaches where females release eggs over gravel at night, accompanied by loud surface splashing known locally as "bulling." Most individuals die after spawning, though some survive to spawn in subsequent years. Eggs and larvae drift downstream, with juveniles feeding on zooplankton and insect larvae in nursery habitat before migrating to sea. As both predator and prey, the species links marine and freshwater food webs and its migratory runs once served as a seasonal feature of river ecosystems.