Age of migration for spawners from three cohorts of allis shad (Alosa alosa) in the Gironde watershed (France)

The population of allis shad (Alosa alosa) of the Gironde watershed is still abundant. The abundance and the structure were estimated for the runs 1994, 1995 and 1996. These estimations are based on surveys and samplings (i) of the captures of the fisheries and (ii) of the passages in fish lift. The strength of the cohorts 1989, 1990 and 1991 was evaluated according to the follow-up of monthly abundance of juveniles in estuary. With this information, it was possible, for the cohorts 1989 to 1991, to calculate the ratio between the number of spawner of one sex, migrating at one age and the number of young of the year (yoy) in estuary. Maximum of the spawners, males as females, migrate at age 5. On average, 5.26 % of the yoy in estuary will become 5 years old female spawners of and 4.89 % 5 years old male spawner. The migration pattern for the adults of 4 and 6 years is different according to the sex. 2.61 % of yoy in estuary will become 4 years old male spawners and 0.73 % 6 years old males. 0.99 % and 2.08 % of the juveniles in estuary will respectively give females of 4 and 6 years. The entrance of 3 years and 7 year old spawners remain lower than 0.2 % for the two sexes. Globally, 8.40 % of the yoy in estuary become female spawners, 8.33 % male spawners. Finally, notice the strong variability of the total proportion of female produced by a cohort. 51.0 % of the spawners from the cohort 1989 were females, 56.8 % from the cohort 1990 and only 40.9 % from the cohort 1991.Original Abstract: La population de grandes aloses (Alosa alosa) du bassin versant de la Garonne est encore abondante. Laimportance et la structure des flux de migration 1994, 1995 et 1996 ont ete estimees a partir du suivi des captures de la pecherie et des passages au niveau daun obstacle. Des campagnes daechantillonnage precisent les caracteristiques biologiques des individus. La force des cohortes 1989, 1990 et 1991 est basee sur le suivi des abondances mensuelles des alosons en estuaire. A partir de ces informations, il a ete possible, pour les cohortes 1989 a 1991, de rapporter les effectifs de geniteurs daun sexe donne, remontant a un age donne au nombre daalosons presents en estuaire lors de leur premiere annee. Caest a 5 ans que remonte le maximum de geniteurs, males comme femelles. En moyenne, 5,26 % des alosons en estuaire donnent des femelles de 5 ans et 4,89 % des males de 5 ans. Le patron de migration pour les geniteurs de 4 et 6 ans differe selon le sexe : 2,61 % des alosons