Shad are herring-like fish that spend most of their adult lives in the sea but spawn in rivers (or, occasionally, in the upper reaches of estuaries) and usually migrate through estuaries in spring months on their way to the spawning grounds. The simplest way to distinguish between the two species is by their size (Allis shad, Alosa alosa, are usually 30-50cm, Twaite shad, Alosa fallax, 25-40cm), the number of scales running along the lateral line, and the number of gill rakers on the first gill arch.
There are few records of shad in the SAC. Both are considered to be nationally rare and vulnerable, the Allis shad more so than the Twaite. Data is needed to confirm whether these species are indeed still present within the site, and whether they are breeding. Nationally the Environment Agency, Countryside Council for Wales and English Nature are involved in a joint work programme to identify key rivers and spawning sites for both species.
Both species are vulnerable to fishing (as a bycatch), pollution, and river obstructions to migration (especially as their deep keeled body prevents them from using fish passes). The Allis shad is particularly vulnerable to migration obstruction as it spawns further upriver than the Twaite. Both species are included in Section 9 (4) (a) of the Wildlife and Countryside Act (1981), (amended April 1998), which makes it an offence to intentionally obstruct access to spawning areas or to damage or destroy gravels used for spawning. The Twaite shad is also protected under Section 5 of the Wildlife and Countryside Act (1981). Both species are Priority Species in the UK Biodiversity Action Plan.