Twaite Shad

The Twaite Shad (Alosa fallax) is a medium-sized anadromous fish in the herring family, found along Atlantic and Mediterranean coasts of Europe and North Africa. Smaller and more coastal than its relative the Allis Shad, it spends most of its adult life in shallow marine and estuarine waters, entering rivers in spring to spawn. It is an important indicator of estuarine and river health, since its migratory life cycle requires open passage between sea and freshwater. Some landlocked, non-migratory populations persist in lakes such as Italy's Lake Garda. Historic declines from river obstruction and pollution have led to protected status in several European countries, with restoration efforts focused on fish passage.

Twaite Shad have a slim, silvery, laterally compressed body typical of the herring family, with an olive-green back and a forked tail.

• Distinctive row of 5-8 dark spots running along the upper flank behind the head, more numerous than the Allis Shad's single spot
• Deeply notched upper jaw and a toothed keel of scutes along the belly
• Coarser, fewer gill rakers (30-60) than the Allis Shad
• Typical adult length 30-50 cm, smaller on average than the Allis Shad
• Large, silvery cycloid scales that detach easily when handled

Twaite Shad inhabit shallow coastal marine waters, estuaries, and lower river reaches across the eastern Atlantic from Scandinavia to Morocco and throughout the Mediterranean. They tolerate a wide salinity range, moving freely between saltwater and brackish estuarine zones, and ascend rivers only a short distance to spawn compared with the Allis Shad. Spawning takes place in spring over gravel or sandy river shallows just above the tidal limit. A few non-anadromous, freshwater-resident populations exist in large lakes such as Lake Garda and Lake Bourget, having become landlocked after post-glacial isolation. Juveniles use estuarine nursery grounds before dispersing to coastal waters.

Twaite Shad school in open water, filter-feeding on zooplankton and small fish using fine gill rakers. During spring spawning runs, adults gather near river mouths and move only short distances upstream, spawning at night over shallow gravel with vigorous surface activity. Unlike the Allis Shad, many Twaite Shad survive spawning and can reproduce in multiple years. Juveniles rear in estuarine nurseries, feeding on plankton before moving to coastal waters as they mature. The species plays a role as both predator of small invertebrates and prey for larger fish and birds, and its seasonal river runs remain notable in parts of its range.