Theallis shad(Alosa alosa) is a widespread Northeast Atlantic species offishin theAlosidae family.It is ananadromousfish which migrates into fresh water to spawn.[1][3]It is found in the eastern Atlantic Ocean, the western Baltic Sea and the western Mediterranean Sea. In appearance it resembles anAtlantic herringbut has a distinctive dark spot behind the gill cover and sometimes a row of up to six spots behind this. It sometimeshybridiseswith thetwait shad(A. fallax). This fish becomes mature when three or more years old and migrates to estuaries, later swimming up rivers to spawn. Populations of this fish have declined due tooverfishing,pollution andhabitat destruction.Conservation of this species is covered by Appendix III of theBern Conventionand Appendix II and V of theEuropean Community Habitats Directive.
| Allis shad | |
|---|---|
| |
Scientific classification
| |
| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Actinopterygii |
| Order: | Clupeiformes |
| Family: | Alosidae |
| Genus: | Alosa |
| Species: | A. alosa
|
| Binomial name | |
| Alosa alosa | |
| Synonyms[1][2][3] | |
| |
Description
editThe allis shad is a typical herring-type fish. It has nolateral lineand a somewhat rounded belly. Thegill coveris ridged and thescaleslarge. The back is a bluish-green colour and the head brownish with a golden tinge on theoperculum.The flanks are silvery, sometimes with a bronzy tinge, and a distinctive large dark spot occurs just behind the gill cover, and occasionally one to six smaller spots behind that. The adult length is typically 30 to 60 cm (12 to 24 in).[3][4]
Distribution
editThe allis shad is found in the eastern Atlantic in waters bordering most ofEuropeand northwesternAfrica,and it enters to the westernBalticand westernMediterraneanSeas, but it is rare outside and increasingly withinFrance.[1][3]There are no longer breeding populations in Morocco or Mediterranean Spain.[1]
Biology and lifecycle
edit
Alosa alosahas a similar lifecycle to that of thetwait shadA. fallax.They are known to live insympatry,[5]and the two species can hybridize.[6]They areanadromousspecies like many other species in the genusAlosa.[6]However, some record of them being landlocked suggests an ability to adapt well to their environment.[6] They primarily live at sea on feeding grounds and migrate to their spawning grounds between April and June once they are sexually mature.[6]Maturity usually ranges from 3–7 years of age.[6]A. alosacan usually only reproduce once in their lifetimes.[6]Juveniles appear inestuariesand brackish water around July to August.[5]The salinity of brackish water may pose problems to the juveniles migrating from fresh water.[5]
The estuarine phase, or the time spent in estuaries migrating from spawning grounds to sea, is estimated to have a duration inA. alosaof up to six months.[5]The estimate, however, does not take into account individual variation and/or survival of juveniles in the estuarine phase.[5]
Population reduction
editPopulations have been reduced primarily by overfishing, pollution, and habitat destruction.[6]Siltationandgravelmining threaten spawning locations.[1]There is concern that barriers to migration like dams are causing allis shad to spawn downstream from their usual locations, potentially prompting hybridization with twaite shad.[1]Climate change may force the species' distribution to shift or contract further.[1]TheInternational Union for Conservation of Naturelisted the species ascritically endangeredin 2024, citing an estimated global population decline of around 80% over the past 20 years, leaving many small relict subpopulations andextirpatingothers.[1]
Conservation
editFour special areas of conservation have been designated in Ireland whereAlosaspecies have been known to spawn.[6]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."[5]However,A. alosais currently under a moratorium (2008) in numerous French watersheds.[5]
References
edit- ^abcdefghiFord, M. (2024)."Alosa alosa".IUCN Red List of Threatened Species.2024:e.T903A15541267.Retrieved28 October2024.
- ^"Alosa alosa(Linnaeus, 1758) ".Global Biodiversity Information Facility.Retrieved29 October2024.
- ^abcdFroese, Rainer;Pauly, Daniel (eds.)."Alosa alosa".FishBase.October 2024 version.
- ^"Allis shad:Alosa alosa".NatureGate.Retrieved14 December2013.
- ^abcdefgLochet, 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.
- ^abcdefghCoscia, I., V. Rountree, J. J. King, W. K. Roche, and S. Mariani.A Highly Permeable Species Boundary between Two Anadromous Fishes.Journal of Fish Biologydoi:10.1111/j.1095-8649.2010.02768.x77.5 (2010): 1137-149.
External links
edit
Data related toAlosa alosaat Wikispecies- EU LIFE-Project: The re-introduction of allis shad (Alosa alosa) in the Rhine system
- .New International Encyclopedia.1905.
