The Shads - Twaite and Allis

Two species of shad occur in Scottish waters - the Twaite shad (Alosa fallax) and the Allis shad (Alosa alosa). So similar are they in physical appearance, that to anyone other than a specialist, it is very difficult to physically differentiate between these two species. This is made even more difficult because there is some suggestion that both species hybridise.

Are they truly freshwater species?

No - both species are anadromous, meaning that they live as adults in the sea, but migrate to freshwater to spawn between April-May. After spawning, and once the eggs have hatched, the juvenile fish migrate to sea over the next few months. These fish feed on a range of marine crustaceans and small fish before returning to the river to spawn 3-4 years later, after they reach maturity.

Where can they be found?

In the British Isles, both species are more commonly associated with the south-western coastline, and, in particular, the rivers Severn, Usk and Wye. In Scotland, they are more commonly associated with the Solway Estuary, although little is known about their adult distribution and spawning sites. Within Europe, shad are present in coastal environments throughout extending from southern Norway southwards towards the eastern Mediterranean.

Are they under threat?

Both species spawn on rocky river substrates and require good quality, well oxygenated rivers to allow the eggs to survive. Water quality and access to suitable spawning areas is therefore a significant issue for these fish. Shad were previously exploited for food and, in other parts of its British and European range, both Twaite and Allis shad supported a number of commercial fisheries. Although it is now illegal to fish for shad in Britain, some continental European fisheries still remain in operation.

What is their conservation status?

Both Twaite and Allis shad are protected via Schedule 5 of the Wildlife & Countryside Act 1981, and are listed on Annexes II and V of the EU Habitats Directive. They are also contained in Appendix III of the Bern Convention and are on the UKBAP Priority List.