The twait shad or twaite shad (''Alosa fallax'') is a species of

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of li ...

in the family

Clupeidae Clupeidae is a family of ray-finned fishes, comprising, for instance, the herrings, shads, sardines, hilsa, and menhadens. The clupeoids include many of the most important food fishes in the world, and are also commonly caught for production of ...

. It is found in the eastern Atlantic Ocean and the Mediterranean Sea and is an

anadromous fish Fish migration is mass relocation by fish from one area or body of water to another. Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousan ...

which lives in the sea but migrates into fresh water to spawn. In appearance it resembles an

Atlantic herring Atlantic herring (''Clupea harengus'') is a herring in the family (biology), family Clupeidae. It is one of the most abundant fish species in the world. Atlantic herrings can be found on both sides of the Atlantic Ocean, congregating in large ...

but has a row of six to ten distinctive spots on its silvery flanks. They become mature when three or more years old and migrate to estuaries, later swimming up rivers to spawn. Populations of this fish have declined due to

overfishing Overfishing is the removal of a species of fish (i.e. fishing) from a body of water at a rate greater than that the species can replenish its population naturally (i.e. the overexploitation of the fishery's existing fish stock), resulting in th ...

pollution Pollution is the introduction of contaminants into the natural environment that cause adverse change. Pollution can take the form of any substance (solid, liquid, or gas) or energy (such as radioactivity, heat, sound, or light). Pollutants, the ...

and

habitat destruction Habitat destruction (also termed habitat loss and habitat reduction) is the process by which a natural habitat becomes incapable of supporting its native species. The organisms that previously inhabited the site are displaced or dead, thereby ...

. Conservation of this species is covered by Appendix III of the Bern Convention and Appendix II and V of the European Community Habitats Directive.

Description

The twait shad is a typical herring-type fish and much resembles the

allis shad The allis shad (''Alosa alosa'') is a widespread Northeast Atlantic species of fish in the Clupeidae, herring family, Clupeidae. It is an Fish migration, anadromous fish which migrates into fresh water to spawn. It is found in the eastern Atlan ...

. It has no

lateral line The lateral line, also called the lateral line organ (LLO), is a system of sensory organs found in fish, used to detect movement, vibration, and pressure gradients in the surrounding water. The sensory ability is achieved via modified epithelial ...

and the belly is more rounded than that of the

sprat Sprat is the common name applied to a group of forage fish belonging to the genus ''Sprattus'' in the family Clupeidae. The term also is applied to a number of other small sprat-like forage fish (''Clupeoides'', ''Clupeonella'', '' Corica'', ' ...

and

Baltic herring Atlantic herring (''Clupea harengus'') is a herring in the family Clupeidae. It is one of the most abundant fish species in the world. Atlantic herrings can be found on both sides of the Atlantic Ocean, congregating in large schools. They can ...

. The

gill cover The operculum is a series of bones found in bony fish and chimaeras that serves as a facial support structure and a protective covering for the gills; it is also used for respiration and feeding. Anatomy The opercular series contains four bon ...

is ridged and the

caudal peduncle Fins are distinctive anatomical features composed of bony spines or rays protruding from the body of a fish. They are covered with skin and joined together either in a webbed fashion, as seen in most bony fish, or similar to a flipper, as se ...

has large, plate-like

scales Scale or scales may refer to: Mathematics * Scale (descriptive set theory), an object defined on a set of points * Scale (ratio), the ratio of a linear dimension of a model to the corresponding dimension of the original * Scale factor, a number w ...

. 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 .

Distribution

The twait shad is found in most of Europe and all countries bordering the Mediterranean Sea.

Biology and lifecycle

''Alosa fallax'' has a similar lifecycle to ''A. alosa'' (

). They are known to live in

sympatry In biology, two related species or populations are considered sympatric when they exist in the same geographic area and thus frequently encounter one another. An initially interbreeding population that splits into two or more distinct species sh ...

.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. Some studies have suggested ''A. fallax'' and ''A. alosa'' species can hybridize. They are

anadromous Fish migration is mass relocation by fish from one area or body of water to another. Many types of fish migrate on a regular basis, on time scales ranging from daily to annually or longer, and over distances ranging from a few metres to thousan ...

species, like many other species in the genus ''Alosa''.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'' 77.5 (2010): 1137-149. However, some record of them being landlocked suggests an ability to adapt well to their environment. They primarily live at sea on feeding grounds and migrate to their spawning grounds between April and June once they are sexually mature. Maturity usually ranges from 3–7 years of age. Juveniles appear in

estuaries An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environment ...

and brackish water around June to July. The salinity of brackish water may pose problems to the juveniles migrating from fresh water.

Population reduction

Populations have been reduced primarily through overfishing, pollution, habitat destruction and migratory route obstruction. 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 ''A. fallax'' of up to a year and a half. The estimate, however, does not take into account individual variation and survival of juveniles in the estuarine phase.

Conservation

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." In England the Unlocking The Severn project, a consortium of the

Canal & River Trust The Canal & River Trust (CRT), branded as in Wales, holds the guardianship of 2,000 miles of canals and rivers, together with reservoirs and a wide range of heritage buildings and structures, in England and Wales. Launched on 12 July 2012, the ...

, Severn Rivers Trust,

Environment Agency The Environment Agency (EA) is a non-departmental public body, established in 1996 and sponsored by the United Kingdom government's Department for Environment, Food and Rural Affairs, with responsibilities relating to the protection and enha ...

and

Natural England Natural England is a non-departmental public body in the United Kingdom sponsored by the Department for Environment, Food and Rural Affairs. It is responsible for ensuring that England's natural environment, including its land, flora and fauna, ...

is creating fish passes around weirs on the

River Severn , name_etymology = , image = SevernFromCastleCB.JPG , image_size = 288 , image_caption = The river seen from Shrewsbury Castle , map = RiverSevernMap.jpg , map_size = 288 , map_c ...

. The aim is to open 150 miles (241 km) of the river for the fish to increase its freshwater breeding habitat. By 2021 passes at Diglis and Bevere, near Worcester, have been completed.

References

External links

* {{Taxonbar, from=Q217759

twait shad The twait shad or twaite shad (''Alosa fallax'') is a species of fish in the family Clupeidae. It is found in the eastern Atlantic Ocean and the Mediterranean Sea and is an anadromous fish which lives in the sea but migrates into fresh water to s ...

Fish of the Black Sea Fish of the Mediterranean Sea Marine fauna of North Africa Marine fish of Europe Taxa named by Bernard Germain de Lacépède Taxonomy articles created by Polbot