Reproductive biology of male skipjack tuna Katsuwonus pelamis (Linnaeus) in the tropical western and central Pacific Ocean

The reproductive biology of male skipjack tuna Katsuwonus pelamis was examined from May 2005 to December 2007 in the tropical western and central Pacific Ocean. Testis maturity was classified into five stages (i.e., immature, early maturation, mid maturation, late maturation, and regression) on the basis of morphological changes in the germinal epithelium. The testis duct system (TS), which was characterized by lobules that lack spermatocysts, store only sperm, and form an anastomosing network, was located under the main sperm duct. The TS was observed at all stages of maturity except the immature stage, and first appeared in individuals with a fork length (FL) of 38-40 cm. The estimated minimum size at first maturity was 35.5 cm FL and the size at 0.5 maturity was 40.7 cm in FL. It was difficult to clearly differentiate the testis maturity stage based solely on the gonad index (GI) class because each GI class contained mature fish. Monthly changes in testis height index and GI did not follow a seasonal cycle. Mature individuals dominated throughout the year. These results imply that male skipjack tuna in this area showed evidence of spawning activity throughout the year without a clear seasonal pattern.