Twaite shad
Alosa agone
The average adult size is 40-50 cm and a maximum weight of about 1 kg. It has a very compressed body laterally. The eyes are large and have adipose membranes. The body is silvery with bluish hues on the back. There are some black spots behind the operculum. It differs from the allis shad in that it has less than 60 gill spines in the gill arches and less than 60 scales on the longitudinal axis of the body.
Genetic differences between Atlantic twaite shad (Alosa fallax) and the Mediterranean one (Alosa agone) suggest the latterit can be considered an independent species. In Catalonia, twaite shad is currently present in the rivers Ebro and Fluvià, but in the past it was also found in the Llobregat and the Ter, where it disappeared due to the multiple environmental impacts they suffer.
The twaite shad is an anadromous species. Adults live in the sea and go up rivers for breeding. They feed on plankton and other invertebrates, and adults also on small fish.
Distribution
Allis shad
Alosa alosa
The allis shad, when adult, measures 30 to 60 cm and with weights up to 2 kg. The eyes have adipose membranes. It has a silvery coloration with bluish tones on the back. There is usually a black spot behind the operculum. It is distinguished from the twaite shad by having more than 80 gill spines in the gill arches and more than 70 scales in the longitudinal series.
It is more abundant on the Atlantic coasts, but it is rarer on the Mediterranean side where many populations have become extinct. In Catalonia it was present in the river Ebro, but the barrier of the Xerta weir, among other impacts such as pollution and overfishing, caused that it was diminishing progressively until its disappearance during the decade of 1960.
It is an anadromous species. Adults live in the sea and enter rivers to spawn in the spring. They feed on invertebrates, mainly planktonic crustaceans, and occasionally small fish.
Distribution
