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) in early summer. The number of allis and twaite shad are not considered to be stable. 

Allis shad Alosa alosa has been designated as a qualifying feature in Carmarthen Bay and Estuaries SAC as it is considered an important coastal migration route or feeding ground, and as the upstream catchment (River Tywi SAC) has historically supported a substantial spawning population. There may be other relevant rivers that contribute to the SAC population.

The species is currently (2025) classified as Critically Endangered (presumed extinct) in Wales. 

Twaite shad Alosa fallax has also been designated as a qualifying feature in the Carmarthen Bay and Estuaries SAC. Twaite shad migrate through the Carmarthen Bay and Estuaries SAC to reach spawning sites in the River Tywi SAC, and are likely to use the inshore coastal waters of Carmarthen Bay for feeding, and the Three Rivers estuary for nursery areas. There may be other relevant rivers that contribute to the SAC population.