PLASMA VITELLOGENIN AND 17-BETA-ESTRADIOL CYCLES IN THE SKIPJACK TUNA(KATSUWONUS-PELAMIS) OF THE WESTERN INDIAN-OCEAN

Vitellogenin (VTG) levels were measured in the plasma of 143 female sk ipjack tuna, Katsuwonus pelamis caught in the Western Indian Ocean. Th ese levels were con-elated with the gonadosomatic index (GSI) and with plasma 17 beta-estradiol (E-2) levels during the reproductive cycle. VTG and E-2 levels were measured using a competitive enzyme-linked imm unosorbent assay (ELISA) and a radioimmunoassay (RIA), respectively. V TG was purified from a pool of plasma obtained from females with high GSI using a double chromatography method (gel filtration and ion excha nge). A specific antibody was obtained in rabbits. The VTG immunoenzym eassay developed gave an assay detection limit (90 % binding) of 15 ng .ml(-1). Mean VTG levels ranged from 1.2 mg.ml(-1) to 4 mg.ml(-1). Mea n monthly VTG values remained relatively elevated during the resting p eriods (1.2 mg.ml(-1) in April and 1.9 mg.ml(-1) in September), while maximum levels reached only 4 mg.ml(-1) during the reproductive season . The highest GSI values were observed from November to March during t he major reproductive season corresponding to the North monsoon and fr om early June to late August during the minor reproductive season corr esponding to the East monsoon. Lowest GSI values were found in April-M ay and September-October. Mean E, levels exhibited considerable variab ility among all females sampled ranging from 700 pg.ml(-1) to 9 ng.ml( -1). This study demonstrated that the sexual maturation was correlated with the monsoon seasons and that there was a positive correlation be tween monthly variations of GSI, VTG and E-2 levels in the skipjack tu na population from the Western Indian Ocean.