Tosashimizu Fisheries Guidance Station, Kochi Prefectural Government, Kochi 787-0333, Japan
Changes in ovarian development of the wild bullet tuna, which were collected off Cape Ashizuri during the daytime (0500-1500h) at two-hour intervals June 12-14, 1997, were examined to understand its ovarian maturation cycle and batch fecundity. The changes in diameter of the most advanced oocyte group, GSI, and occurrence of egg-ovulation in the ovarian cavity showed that hydration (oocyte maturation) occurred between 1100 and 1300h, ovulation at around 1500h, and thereafter, eggs were spawned. Hydrated ovary collected at 1100h had three clutches of oocytes, their modes of diameter were distributed at around 0.29~0.3mm, 0.41~0.42mm, and 0.6-0.81mm, respectively. It was estimated that the oocyte group with 0.29~0.3mm diameter developed to0.41~0.42mm in diameter after 24hours, and finally reached the maturation stage of 0.6-0.81mm in diameter after 48hours. The batch fecundity (BF) was calculated from the size of the single clutch of hydrated oocytes, and related to the body weight (RBW) excluding both gonad and stomach content weight as the following formula, BF=222RBW+672 (r2= 0.2506, n=52).