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Twaite Shad (ALOSA FALLAX)


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SHAD, TWAITE / ALOSA FALLAX
*AT AT 1.41 3.2 Deal England 49 T. Hayward
*AT AT 1.41 3.2 Torbay England 54 S. Jenkins
*SH SH 1.24 2.12 Garlieston Scotland 78 J. Martin

The twaite shad (ALOSA FALLAX)

is a member of the herring family. It is difficult to distinguish from its close relative 1102 Allis shad Alosa alosa. Both fish have streamlined bodies covered with distinct, large, circular scales which form a toothed edge on the lower margin and an adipose membrane which partially covers each eye. Rarely exceeding 40 cm length, twaite shad are usually smaller than allis shad, which measure 30-50 cm. However, the only reliable way of separating the two species is to examine the gills – twaite shad have only 40-60 gill-rakers (comb-like structures that are used to filter zooplankton) on the first gill arch, whereas allis shad have 90-130.

The twaite shad Alosa fallax is found along the western coastline of Europe, from southern Norway to Morocco and along the eastern Mediterranean, but has declined substantially throughout Europe.

twaite shad (ALOSA FALLAX)

In the UK, spawning stocks of twaite shad Alosa fallax are known to occur in only a few rivers in Wales and on the England/Wales border, flowing into the Severn estuary (Carstairs 2000); no spawning stocks are known north of this, although the species is present in south-west Scotland, in rivers flowing into the Solway Firth, where hybrids with 1102 Allis shad Alosa alosa have been reported (Maitland & Lyle 2001).

This species returns from the sea to spawn in spring, usually between April and June, hence the alternative name of ‘May fish’. The habitat requirements of twaite shad are not fully understood. On the River Usk and the River Wye, twaite shad are known to spawn at night in a shallow area near deeper pools, in which the fish congregate. The eggs are released into the water column, sinking into the interstices between coarse gravel/cobble substrates. The majority of adults die after spawning, though UK populations appear to have an unusually high proportion of repeat spawners – up to 25%. After hatching the fry develop and slowly drift downstream. Recruitment seems to be highest in warm years, and high flows between May and August may result in fry being washed prematurely out to sea.

Population declines in many parts of Europe have been attributed to pollution, overfishing and migratory route obstructions.

Information supplied by http://www.jncc.gov.uk and http://blaw.free.fr

 
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