RECENT DEVELOPMENTS ON BIOLOGICAL AND MOR PHOLOGICAL-CHARACTERISTICS OF TWAITE SHAD (ALOSA-FALLAX-RHODANENSIS ROULE, 1924) POPULATION IN THE RIVER RHONE

Morphological and biological characteristics of the present twaite Sha d (Alosa fallax rhodanensis) population in the River Rhone (France) ar e presented. Results are obtained from analysis of a 1479 adults sampl e collected during upstream migration over three years. Used morpholog ical characters (mainly the number of gillrakers) and their low indivi dual variability reveal that only one taxon is present in the sample a nd is identified as Alosa fallax rhodanensis. Mean length (Lf) and wei ght are 433 mm and 1115 g, respectively. Age is ranged from 3 to 8 yea rs and sex-ratio is on average in favour of females (males/females = 0 .61). Females are older than males (5.24 years against 4.21 years for mean migration age (AMR) and get a better growth in size and weight at the same age. Mean first reproduction age (AMPR) is lower than AMR be cause of a large iteroparity phenomenon. AMPR is higher in females (4. 00 years) than in males (3.35 years). Number of previous spawnings is ranged from 1 to 5 and mean proportion of previous spawners (at least one time) is 48.6%, but it is higher in females. There exist significa nt interannual differences in the most biological parameters. Present results are discussed and compared to these obtained by DOUCHEMENT (19 81) by taking into account variability related to sampling methods. On one hand, they confirm some biological characteristics of Alosa genus and studied taxon. On the other hand, they suggest a possible change in demographical strategy of Rhone Shad population in 20 years.