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The allis shad is a typical herring-type fish. 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 scales large. 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 is a distinctive large dark spot just behind the gill cover. The adult length is typically 30 to 60 centimetres (12 to 24 in).
Biology and Life-cycle
Alosa alosa has a similar lifecycle to that of the twait shad Alosa fallax. They are known to live in sympatry with Alosa fallax. 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. Alosa alosa can usually only reproduce once in their lifetime. It is observed that juveniles appear in estuaries, brackish water, around July to August. The salinity of brackish water may impose problems to the juveniles migrating from freshwater.
Populations have been reduced primarily by overfishing, pollution, and habitat destruction. Hybridization between species is more likely with species affected by human disturbances. It is estimated that the estuarine phase, or the time that they are in the estuaries migrating from spawning grounds to sea, has a duration in Alosa alosa of up to six months. The estimate however does not take into account individual variation and/or survival of juveniles in the estuarine phase.
Four special areas of conservation have been designated in Ireland where Alosa species have been known to spawn. Alosa alosa "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."  However, Alosa alosa are currently under a moratorium (2008) in numerous French watersheds.
- Freyhof, J. & Kottelat, M. 2008. Alosa alosa. In: IUCN 2011. IUCN Red List of Threatened Species. Version 2011.2. <www.iucnredlist.org>. Downloaded on 16 December 2011.
- "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.