Comparison of biological characteristics of allis shad (Alosa alosa) in wani Sebou (North-east Atlantic coast of Morocco) and in Lima River (Portugal)

Allis Shad (Alosa alosa Linne, 1758) was present throughout the East Atlantic coast from Southern Norway to Morocco. Today the species remains in a few French rivers with rather abundant populations and in some Portuguese streams and Morocco wadi where the stocks decline steadily. The comparison of selective environmental pressures using a comparison of biological characters is a new approach to discriminate sub-populations (Sebou and Lima populations), compared to what was used to describe these populations by their morphometric characteristics. The Alosa alosa populations share several components: (1)the presence of hybrids with Twaite Shad (Alosa fallax), (with an intermediate number of gillrakers) seems related to hydraulic impoundments and to overlapping both species on constraints spawning grounds; (2)an age structure (the 4-6 years age group is dominant) and a similar average age for upstream migration; (3)a similar relative fecundity. Some divergences have been emphasized: (1)a higher average number of gillrakers in the Lima population (modal number 130 vs 110); (2)a better growth of Sebou shads who live in an upwelling zone; (3)a seasonal delay in the migration timing (December in Sebou and February in the Lima river); (4)a seasonal variation of Sebou population sex ratio. Results were discussed and this comparaison showed the existence of one biogeographic unit within Atlantic distribution area which displays homogeneous biological characteristics.