REPRODUCTION OF SKIPJACK TUNA (KATSUWONUS -PELAMIS) FROM THE WESTERN INDIAN-OCEAN

A detailed description of the different stages of oocytes development and the developmental phases of the ovary of the skipjack tuna (Katsuw onus pelamis) are provided during several annual reproductive cycles. Skipjack tuna were sampled from commercial catches in two distinct are as of the Western Indian Ocean from February 1989 to February 1994. Sa mples were collected, either on board tuna purse seiners (1656 fish), or at the tuna cannery of Mauritius (4387 fish). Size at first maturit y for females is 41-42 cm fork-length, and for males 42-43 cm, corresp onding approximately to 1.5-year-old fish. The monthly variation of th e proportion of different maturity stages of gonads assessed by visual examination shows that, whatever the month, there are always 70% of s ampled females which have ovaries in the terminal stage of maturation (stage IV). The variations of the, gonadosomatic index indicate that s pawning of this species occurs all year round with some periods of mor e intense sexual activity. Histological examination of 737 ovaries ind icates that postovulatory follicles (indicator of a recent spawning) a re basically present during the two monsoon seasons, Northwest monsoon (from November to March) and Southeast monsoon (from the beginning of June to the end of August). The percentage of ovaries with atretic fo llicles is maximun during the two intermonsoon seasons (April-May and September-October). For all the period covered by this study and for t he two sampled areas, the number of males is significantly greater tha n the number of females. The monthly variations of sex-ratio indicate that the predominance of males is more pronounced during the intense s pawning periods.