1102 Allis shad Alosa alosa
Description and ecological characteristics
The allis shad Alosa alosa is a member of the herring family. It is difficult to distinguish from its close relative 1103 Twaite shad Alosa fallax. 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. Measuring 30-50 cm, allis shad are usually larger than twaite shad, which rarely exceed 40 cm. However, the only reliable way of separating the two species is to examine the gills – allis shad have 90-130 gill-rakers (comb-like structures that are used to filter zooplankton) on the first gill arch, whereas twaite shad have only 40-60.
Relatively little information is available on the habitat requirements of allis shad in freshwater. It grows in coastal waters and estuaries but migrates into rivers to spawn, swimming up to 800 km upstream in continental Europe. However, allis shad do not readily traverse obstacles to migration such as dams or weirs, and this has been a major cause of their decline. Adults spawn at night with a great deal of noisy splashing; the eggs are released into the current where they settle among gaps in gravelly substrates. Spawning sites tend to be used year after year, and relatively shallow gravelly areas adjacent to deep pools are thought to represent optimal spawning habitat. Almost all adults die after spawning.
Population declines in many parts of Europe have been attributed to the effects of pollution, overfishing and river obstructions to migration.
European status and distribution
The allis shad Alosa alosa is rare and declining throughout its range on the western coasts of Europe, from southern Norway to Spain, and in the Mediterranean eastwards to northern Italy. The most successful breeding populations are thought to be in a few rivers in western France and Portugal.
UK status and distribution
Allis shad Alosa alosa are rare in the UK. Although formerly known to spawn in several British river systems, the only recently-confirmed spawning site is in the Tamar Estuary (Plymouth Sound and Estuaries cSAC). There is probably a spawning population in the Solway Firth area (Maitland & Lyle 2001), but rivers in the Severn catchment may no longer support viable breeding populations (Carstairs 2000).
Site selection rationale
Sites in the UK have been selected where allis shad Alosa alosa has been reliably recorded as present, where there is previous evidence of breeding, and where there still appear to be favourable conditions for breeding. However, because there is only one recently-confirmed spawning population in the UK, this species is not currently a primary reason for site selection at any UK SAC. Marine sites that are considered important migration routes or feeding grounds for this species have also been included within the SAC series, usually where they are adjacent to a freshwater site.
SACs where this Annex II species is a qualifying feature, but not a primary reason for site selection
- Afon Tywi/ River Tywi West Wales and The Valleys
- Carmarthen Bay and Estuaries/ Bae Caerfyrddin ac Aberoedd East Wales, Extra-Regio, West Wales and The Valleys
- Pembrokeshire Marine/ Sir Benfro Forol Extra-Regio, West Wales and The Valleys
- Plymouth Sound and Estuaries Cornwall and Isles of Scilly, Devon, Extra-Regio
- River Usk/ Afon Wysg East Wales, West Wales and The Valleys
- River Wye/ Afon Gwy East Wales, Gloucestershire, Wiltshire and Bristol/Bath area, Herefordshire, Worcestershire and Warwickshire, West Wales and The Valleys
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