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 (Katsuwonus pelamis) are provided during several annual reproductive cycles. Skipjack tuna were sampled from commercial catches in two distinct areas of the Western Indian Ocean from February 1989 to February 1994. Samples were collected, either on board tuna purse seiners (1656 fish), or at the tuna cannery of Mauritius (4387 fish). Size at first maturity for females is 41-42 cm fork-length, and for males 42-43 cm, corresponding approximately to 1.5-year-old fish. The monthly variation of the proportion of different maturity stages of gonads assessed by visual examination shows that, whatever the month, there are always 70% of sampled females which have ovaries in the terminal stage of maturation (stage IV). The variations of the, gonadosomatic index indicate that spawning of this species occurs all year round with some periods of more intense sexual activity. Histological examination of 737 ovaries indicates that postovulatory follicles (indicator of a recent spawning) are 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 follicles is maximun during the two intermonsoon seasons (April-May and September-October). For all the period covered by this study and for the two sampled areas, the number of males is significantly greater than the number of females. The monthly variations of sex-ratio indicate that the predominance of males is more pronounced during the intense spawning periods.