The allis shad, belonging to the herring family, has a blue-green back and silvery white sides and belly. The top edge of its gill cover has a black spot, which is usually followed by another one or two less conspicuous spots. It has no lateral line on its sides. In May and June it journeys upriver to spawn, often traveling great distances against the current. Its eggs float above the river bed and the embryos hatch in 3 to 4 days. When the fish is about 8 to 12 cm long, it migrates from the river to the sea. Here it lives several years and feeds on crustaceans. Reaching a length of 30 to 40 cm, it migrates for the first time to spawn in freshwaters. It lives close to the European coasts of the Atlantic Ocean and in the western part of the Mediterranean.
The twaite shad is a migratory herring species, which enters the lower reaches of rivers to spawn in June and July. Its reproductive process and way of life is similar to the former species, although this fish is smaller. It also has a number of black blotches in a line along the sides. During migration it is most numerous in the lower reaches of European rivers. It lives close to the European shoreline in the Mediterranean, in the Atlantic Ocean and in the Baltic Sea as far as the shores of southern Norway, Sweden and Finland. The allis shad and the twaite shad used to be economically important fishes, but today are usually very rare in the majority of European rivers, the allis shad being especially rare.
Similar shads occur on the American Atlantic coast, namely the alewife (A. pseudoharengus) and the American shad (A. sapidissima) which has been introduced to the Pacific coast.
Maximum size and weight: 70 cm, 2.5 kg.
Identifying characteristics: A black spot behind the upper edge of the gill cover, sometimes another one or two less conspicuous spots