1Graduate School of Marine Science and Technology, Tokai University, Shimizu, Shizuoka 424-8610, 2National Research Institute of Far Seas Fisheries, Fisheries Research Agency, Shimizu, Shizuoka 424-8633, Japan
The ovaries of 395 skipjack tuna Katsuwonus pelamis females caught by purse seine from October 2003 to January 2005 in the tropical region of the Western Central Pacific Ocean were studied by histological observation and classified into the ovarian maturity phases. First mature individuals began to appear at 40 cm in fork length (FL) and 50% maturity was 47.9 cm in FL. The appearance time of the final mature phase was between 15:00 to 21:00. The ovarian maturity phase should not be distinguished by the gonad index because it is not correctly reflected in each maturity phase. While pre-nucleus oocyte occupied 80% or more of the ovary on average in any maturity phase, yolked oocytes were 5-10%. The final and active mature phase and the post-ovulated phase in the ovary of mature females were observed throughout the year, but the inactive mature phase rarely existed. These results indicate that almost mature skipjack tuna females in this area spawn one night all the year round and they do not have an annual reproductive cycle.