1103 Twaite shad Alosa fallax
Description and ecological characteristics
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.
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.
European status and distribution
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.
UK status and distribution
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).
Site selection rationale
Three river systems have been selected in the Bristol Channel/ Severn Estuary area, where evidence indicates there are extensive areas suitable for spawning, with good prospects for conservation of habitat structure and function. Marine sites that are considered important migration routes or feeding grounds have also been selected.
Afon Tywi/ River Tywi
West Wales and The Valleys
A large spawning population of twaite shad Alosa fallax occurs in the Tywi, south Wales, and is considered to be self-sustaining. Spawning sites occur throughout the lower reaches of the river between Carmarthen and Llangadog, with most spawning occurring downstream of Llandeilo. Water quality and quantity are considered adequate to maintain this internationally vulnerable species, and there are no impassable obstructions along the migration route, though one weir at Manorafon may be an obstacle during low flow conditions. The presence of Llyn Brianne reservoir at the headwaters provides the potential to manipulate river flows to aid shad migration.
Carmarthen Bay and Estuaries/ Bae Caerfyrddin ac Aberoedd
East Wales, Extra-Regio, West Wales and The Valleys
Twaite shad Alosa fallax migrate though the waters of Carmarthen Bay and Estuaries cSAC to reach spawning sites in the Afon Tywi. The Taf-Tywi-Gwendraeth estuary is also an important nursery area for juveniles and it is likely that twaite shad feed in the inshore waters of Carmarthen Bay.
River Usk/ Afon Wysg
East Wales, West Wales and The Valleys
The River Usk is one of the largest rivers in south Wales, and twaite shad Alosa fallax has long been known to spawn there. The Usk is one of only four sites in the UK where a known breeding population of twaite shad occurs (the Rivers Wye and Tywi are other SAC sites). Water quality and quantity are considered favourable for this species. The main channel is largely unmodified and a variety of aquatic habitats are present, including good quality spawning gravels and deep pools used for cover by adults and fry. However, Trostrey and Rhadyr Weirs may be a barrier to shad migration under low flow conditions.
River Wye/ Afon Gwy
East Wales, Gloucestershire, Wiltshire and Bristol/Bath area, Herefordshire, Worcestershire and Warwickshire, West Wales and The Valleys
Twaite shad Alosa fallax have long been abundant in the Wye, an extensive river system spanning the border between England and Wales. Twaite shad often spawn at or just above the tidal limit, but in the Wye they migrate over 100 km upstream, the highest spawning site being at Builth Wells. Data held by the Environment Agency indicate that, of the three selected rivers, the largest spawning areas for this species occur on the Wye. The river has relatively good water quality, adequate flows through an unobstructed main channel and a wide range of aquatic habitats conducive to supporting this fish species. In particular, there are a number of deep pools essential for congregation before spawning.
Severn Estuary/ Môr Hafren
Dorset and Somerset, East Wales, Extra-Regio, Gloucestershire, Wiltshire and Bristol/Bath area
Species occurrence description not yet available.
SACs where this Annex II species is a qualifying feature, but not a primary reason for site selection
- Pembrokeshire Marine/ Sir Benfro Forol Extra-Regio, West Wales and The Valleys
Many designated sites are on private land: the listing of a site in these pages does not imply any right of public access.