Shads corresponds to a group of fish species belonging to the same family as sardines and herrings, the Clupeidae. In Europe, especially in France, we find two species that are both diadromous.

They carry their growth at sea and migrate upstream to spawn in freshwater. The Twaite shad (Alosa fallax) is the smallest. It spawns on the downstream part of the rivers, in tidal areas. The Allis shad (Alosa Alosa) goes upper on the streams and spawns in the middle part of the axis, like on the Dordogne in the Bergerac area.

Like all other migratory fishes, shads are very sensitive species. Many factors have led to a worrying decline of their populations, like migratory barriers, overfishing, degradation of habitat quality due to pollution, extraction of sediments and alteration of sediment transport

Thus, the distribution of these two species area has decreased significantly and the last remaining populations is mainly on the Atlantic coast of France and Portugal, and do not present satisfactory conservation conditions.

The same observations could be made about American shad (Alosa sapidissima), originally from the East Coast of the United States, where populations are confronted with comparable issues and evolutions of stocks.