Two species occur in Europe, much resembling each other - one commonly called allis shad (Clupea alosa or Alosa vulgaris), and the other known as twaite shad(Clupea finta or Alosa finta).
So closely allied are these two fishes that their distinctness can be proved only by an examination of the gill-apparatus, the allis shad having from sixty to eighty very fine and long gill-rakers along the concave edge of the first branchial arch, whilst the twaite shad possesses from twenty-one to twenty-seven stout and stiff gill-rakers only.
They inhabit the coasts of temperate Europe, the twaite shad being more numerous in the Mediterranean.
While they are in salt water they live singly or in very small companies, but during May (the twaite shad some weeks later) they congregate, and in great numbers ascend large rivers, such as the Severn (and formerly the Thames), the Seine, the Rhine, the Nile, &c., in order to deposit their spawn.
In the Elbe the twaite shad spawns below Hamburg, the allis shad above Dresden.
The allis shad is caught at a size from 15 to 24 in., and is better flavoured than the twaite shad, which is generally smaller.
The habitat requirements of twaite shad are not fully understood.
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