Sexual differences in the occurrence of Pacific bluefin tuna Thunnus orientalis in the spawning ground, Yaeyama Islands

Pacific bluefin tuna Thunnus orientalis occur in the south-western North Pacific spawning ground from April to July. All females and males occurring there are thought to be involved in the spawning event, but details of their occurrence are not well understood. A daily fishing port survey was conducted at Ishigaki Island (24°21’N, 124°09’E), Yaeyama Islands, during the entire fishing season (April to July) between 2009 and 2014 to assess sexual differences in the timing of occurrence and physiological condition for reproduction of Pacific bluefin tuna. In total, males (n = 566) outnumbered females (481, chi-squared test, P < 0.01) and had a larger body size (mean ± SD = 229 ± 18 cm fork length in male, 220 ± 18 cm in female, Welch’s t-test, P < 0.001). Estimated ages were not different between sexes; 6–25 years for females and 6–24 years for males. Males tended to occur earlier in the spawning ground (median date = 20 May) than females (24 May, Mann–Whitney U-test, P < 0.05). Gonadosomatic indices of both sexes increased from mid April, beginning of the fishing season, had higher values in late May to early-mid June, and then decreased. Fat body index and condition factor of both sexes decreased during the fishing season, and loss of body weight was more prominent in females. Sexual differences in the occurrence in the spawning ground are thought to relate with the reproductive activity and the condition factor.