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Diagnostic Description

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Diagnosis: Body somewhat compressed, moderately deep with depth at pectoral fin less than head length, scutes apparent along belly (Ref. 188). Upper jaw notched, lower jaw fitting into it; no teeth on palatine and vomer; gillrakers fairly short and stout, total 30 to 80, shorter than gill filaments (Ref. 188, 59043). Usually a series of 4-8 black blotches behind gill opening, but sometimes a single blotch, the others faint or absent (Ref. 188, 40476, 59043). Alosa fallax resembles Alosa alosa, which has more and longer gillrakers and at most only 3 dark spots on flank (Ref. 188).Description: Body fusiform, a little compressed, the head wedge-shaped in cross-section (Ref. 2945). Dorsal profile curved (Ref. 59043). Gill cover with radiating bony striae; gillrakers fairly short and stout, total 30 to 80, as long as or shorter than gill filaments; lower gill rakers 20 to 40 (Ref. 188, 2945, 59043). Upper jaw with a distinct notch to receive tip of lower jaw (Ref. 2945). Dorsal fin origin a little before midpoint of body; anal fin origin well behind dorsal fin base; dorsal fin with 4-6 unbranched and 12-16 branched rays; anal fin with 3-4 unbranched and 16-22 branched rays; pectoral fin with 15-17 rays; pelvic fin with 1 unbranched and 8 branched rays (Ref. 2945, 89637). Large, thin cycloid scales; 54-71 scales on a longitudinal line; 16-20 transverse scales; scales regularly arranged on flank (Ref. 51442, 59043, 89637). Belly with a keel of scutes; 18-23 pre-pelvic scutes and 12-18 post-pelvic scutes, a total of 32-41 scutes (Ref. 2945, 89637).Colouration: Deep blue dorsally, becoming greenish brown or golden on the sides and silvery ventrally; usually a series of 4-8 black blotches behind gill opening, sometimes a single blotch (Ref. 188, 59043, 88187).
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Recorder
Crispina B. Binohlan
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Life Cycle

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Adults in the sea begin to congregate near mouths of estuaries in April. Enter estuaries and ascend rivers in May and June when water temperature is between 10-14 °C (Refs. 188, 51442, 59043, 89636). Males begin such movements at 2-3 years, females at 3-4 years (Ref. 59043). Although movement upstream is usually limited to a few kilometres above the brackish zone (Refs. 59043, 89486), spawning has also been reported in non-tidal freshwater areas up to 400 km upstream (Ref. 89637). Gametogenesis occurs in the estuaries. Early arrivals in the rivers are mostly males, with the sex ratio becoming more equal with the later arrivals (Ref. 42360). Spawning movements occur with spring tides and peak when river discharge levels are high (Refs. 89636, 89638). However, when flows are too high, movements upstream become limited (Refs. 89636, 89639). Spawn when water temperature is anywhere between 12-22 °C (Ref. 10541). Move to riverine spawning grounds at night; spawn in large, very noisy schools near surface and leave these areas before daybreak (Ref. 10541). Spawning sites consist of sand and gravel areas with flowing water (Ref. 10541). Spent adults return to the sea and may spawn for 3-4 seasons throughout their lifetime (Refs. 30578, 51442, 59043). Most individuals will have lost 22 % of their body weight after spawning (Ref. 89640). There is some evidence that most individuals return to their natal rivers to spawn (Refs. 10541, 59043). Eggs either drift with the current or sink to the bottom (Ref. 59043, 89641). Eggs hatch after 2-8 days, depending on water temperature (optimal 15-25 °C) (Refs. 35387, 41851). Larvae and juveniles move towards the estuaries and river mouths during their first summer and to the sea at the end of their second year (Ref. 59043). Males mature mainly between the ages of 2-5 years, females between 3-7 years (Ref. 188, 2163, 10541). Length at maturity is between 30-40 cm total length (Ref. 88187).
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Susan M. Luna
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Migration

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Anadromous. Fish that ascend rivers to spawn, as salmon and hilsa do. Sub-division of diadromous. Migrations should be cyclical and predictable and cover more than 100 km.
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Morphology

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Dorsal spines (total): 0; Dorsal soft rays (total): 16 - 22; Analspines: 0; Analsoft rays: 19 - 26; Vertebrae: 49 - 59
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Trophic Strategy

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In freshwater, juveniles feed on planktonic crustaceans, rotifers and insects (Ref. 35387). In estuaries they consume mysids, isopods, and amphipods (Ref. 42360).In the sea, adults feed mainly on small fishes and on crustaceans, including euphausiids (Refs. 188, 42360). Although feeding generally ceases during spawning migrations (Ref. 59043), some male individuals were found to feed on copepods, isopods, fishes and insects (Ref. 89633).
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Biology

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Amphihaline species (Ref. 51442), schooling and strongly migratory, but apparently not penetrating far up rivers (Ref. 188, 6683). Adults are usually found in open waters along the coast (Refs. 59043, 89486); juveniles are usually found along estuaries and near the shore (Ref. 59043), possibly making vertical diurnal movements synchronized with the tides; they remain in estuaries for over one year (Ref. 89630). Several landlocked (lake) non-migratory populations exist (Ref. 10541). Mostly anadromous, entering river mouths in March (Italy) or early June (northenr European rivers) to spawn in or above the tidal reaches; adults probably return to sea not long after spawning (Ref. 188, 6683). Eggs are demersal and widely scattered among sand or gravel on river bed (Ref. 118, 6683). Ichthyophagous, feeds on small fishes and crustaceans, the young taking the fry of herrings, sprats and gobies (Ref. 188, 51442). Females grow faster and are always larger than males of the same age (Ref. 10541). Very locally distributed due to pollution and impoundment of large rivers throughout Europe and most populations declined during the first decade of the 20th century, but seem to have stabilized at a low level since then (Ref. 59043). It has been suggested that members of the genus Alosa are hearing specialists with the American shad (Alosa sapidissima) having been found to detect and respond to sounds up to at least 180 kHz (Ref. 89631). This may aid in predator avoidance (e.g. cetaceans) (Ref. 89632). Hybridization between this species and the allis shad (Alosa alosa) has been reported from the Rhine (Ref. 89633) as well as rivers in France and Algeria (Ref. 10541). There is some evidence that indicates that shad hybrids may reproduce (Ref. 27567).
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Importance

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fisheries: minor commercial; price category: low; price reliability: reliable: based on ex-vessel price for this species
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Twait shad

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The twait shad or twaite shad (Alosa fallax) is a species of fish in the family Clupeidae.[1] 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 spawn. In appearance it resembles an Atlantic herring 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, pollution and habitat destruction. 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. 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).[2]

Distribution

The twait shad is found in most of Europe and all countries bordering the Mediterranean Sea.[1]

Biology and lifecycle

Alosa fallax has a similar lifecycle to A. alosa (allis shad). They are known to live in sympatry.[3] Some studies have suggested A. fallax and A. alosa species can hybridize.[4] They are anadromous species, like many other species in the genus Alosa.[4] However, some record of them being landlocked suggests an ability to adapt well to their environment.[4] They primarily live at sea on feeding grounds and migrate to their spawning grounds between April and June once they are sexually mature.[4] Maturity usually ranges from 3–7 years of age.[4] Juveniles appear in estuaries and brackish water around June to July.[3] The salinity of brackish water may pose problems to the juveniles migrating from fresh water.[3]

Population reduction

Populations have been reduced primarily through overfishing, pollution, habitat destruction and migratory route obstruction.[5][4] Hybridization between species is more likely with species affected by human disturbances.[4] 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.[3] The estimate, however, does not take into account individual variation and survival of juveniles in the estuarine phase.[3]

Conservation

Four special areas of conservation have been designated in Ireland where Alosa species have been known to spawn.[4] 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."[3]

In England the Unlocking The Severn project, a consortium of the Canal & River Trust, Severn Rivers Trust, Environment Agency and Natural England is creating fish passes around weirs on the River Severn. 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.[6]

References

  1. ^ a b c Freyhof, J.; Kottelat, M. (2008). "Alosa fallax". IUCN Red List of Threatened Species. 2008: e.T904A174776207. doi:10.2305/IUCN.UK.2008.RLTS.T904A174776207.en. Retrieved 15 November 2021.
  2. ^ "Allis shad: Alosa alosa". NatureGate. Retrieved 14 December 2013.
  3. ^ a b c d e f 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.
  4. ^ a b c d e f g h 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.
  5. ^ "1103 Twaite shad Alosa fallax - SAC selection species account". jncc.defra.gov.uk. Archived from the original on 2011-07-04.
  6. ^ "Rare fish set for return to River Severn breeding grounds". BBC News. 6 May 2021. Retrieved 6 May 2021.

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Twait shad: Brief Summary

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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 spawn. In appearance it resembles an Atlantic herring 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, pollution and habitat destruction. Conservation of this species is covered by Appendix III of the Bern Convention and Appendix II and V of the European Community Habitats Directive.

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