The twait shad is a typical herring-type fish and much resembles the allis shad. It has no lateral line and the belly is more rounded than that of the sprat and Baltic herring. The gill cover is ridged and the caudal peduncle has large, plate-like scales. This fish is more colourful than the Baltic herring. The back is a bluish green colour and the head brownish with a golden tinge on the operculum. The flanks are silvery, sometimes with a bronzy tinge, and there are a distinctive row of six to ten large dark spot just behind the gill cover though these may fade when the fish is dead. The adult length is typically 25 to 40 cm (10 to 16 in).
The twait shad is found in most of Europe and all countries bordering the Mediterranean Sea.
Biology and Life-cycle
Alosa fallax has a similar lifecycle to Alosa alosa (allis shad). They are known to live in sympatry with Alosa alosa. Some studies have suggested that the Alosa fallax and Alosa alosa species can hybridize. They are anadromous species just like many other species in the Alosa genus. However, there is some record of them being landlocked suggesting an ability to adapt well to their environment. They primarily live at sea on feeding grounds and will migrate to their spawning grounds between April and June once they are sexually mature. Maturity usually ranges from 3–7 years of age. It is observed that juveniles appear in estuaries, brackish water, around June to July. The salinity of brackish water may impose problems to the juveniles migrating from freshwater.
Populations have been reduced primarily through overfishing, pollution, and habitat destruction. Hybridization between species is more likely with species affected by human disturbances. It is estimated that the estruarine phase, or the time that they are in the estuaries migrating from spawning grounds to sea, has a duration in Alosa fallax of up to a year and a half. The estimate however does not take into account individual variation and/or survival of juveniles in the estruarine phase.
Four special areas of conservation have been designated in Ireland where Alosa species have been known to spawn. Alosa fallax "has been placed in Appendix III of the Bern Convention (1979) that lists protected fauna species as well as in Appendix II and V of the European Community Habitats Directive (1992) that list, respectively, species whose conservation requires the designation of special areas of conservation and that are subject to management measures." 
- Freyhof, J. & Kottelat, M. (2008). "Alosa fallax". IUCN Red List of Threatened Species. Version 2013.2. International Union for Conservation of Nature. Retrieved 31 January 2014.
- "Allis shad: Alosa alosa". NatureGate. Retrieved 14 December 2013.
- Lochet, A., S. Boutry, and E. Rochard. Estuarine Phase during Seaward Migration for Allis Shad Alosa Alosa and Twaite Shad Alosa Fallax Future Spawners. Ecology of Freshwater Fish 18 (2009): 323-35.
- Coscia, I., V. Rountree, J. J. King, W. K. Roche, and S. Mariani. A Highly Permeable Species Boundary between Two Anadromous Fishes. Journal of Fish Biology doi:10.1111/j.1095-8649.2010.02768.x 77.5 (2010): 1137-149.