QUELQUES TRAITS BIOÉCOLOGIQUES DES ALOSES DU MAROC ; UN PATRIMOINE CULTUREL ET SOCIO-ÉCONOMIQUE RÉCEMMENT DISPARU.

Abstract

Morrocan Allis Shad and Twaite Shad were the most southern populations of the Eastern Atlantic but the distribution area has been considerably reduced - 700 mt were catched in Sebou’s fisheries until 70’s - by alterations resulting from aquatic pollution, hydro-electric impoundments and industrial installations. Along the Atlantic coast, the Sebou river and its main tributary, the Ouerrha, were the only remaining accessible spawning grounds for spawners until 1991. In 1992 an estuary dam stopped the upriver migration of shads ; a fish pass was buildt, but seemed unsuccessful. Biological and morphological characteristics were studied from a sample of more than 1223 shads, caught between 1978 and 1988. Morrocan populations have specific features and characteristics (very sharp in Allis Shad) as compared to these of northern populations. The A l os a alos a population share a lower modal number of gillrakers (110) whose origine is probably coming from some modulating factors (temperature, size of preys, interspecific trophic competition in the sea). The lower average number of some meristic characteristics (fin ray, scutes) probably reflects the plesiomorphic state of characters. This ancestral state has been emphasized by electrophoretic analysis of twaite’s shad proteins. The average age of female and male is 5,8 and 5 years respectively and the adult scales showed characteristics cicatricial winter rings while a flood mark is present in some juveniles scales.The higher growth of Sebou shads results from the presence of an upwelling zone in sea where trophic resources were abundant. The migration period is longer and starts earlier in Sebou River (December) than in Loire (March). Presently, the Allis Shad has disappeared from morrocan waters since 1992, while it was a great cultural, socio-economic and scientific value interest.