On the Maturation of the Yellowtail Cultured in the Floating Net from the Spawning Season to Post-spawning Period

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The maturation of the yellowtail, <I>Seriola quinqueradiata</I> Temminck and Schlegel, cultured in a narrow floating net set in some coastal areas of southern Japan, Kochi and Oita Prefectures, during one to six years has been studied to obtain the fundamental information on its breeding.In one year old fish, the ovaries have not yet enlarged, about 5 g or less in weight, with numerous immature eggs belonging to early peri-nucleolus stage, throughout the spawning season.Male, however, possesses ripe testes from April to the beginning of May, though not enlarged markedly.In 3 to 6 years old fish, the gonads are well enlarged, nearly reaching 400g to 500g in weight in April, but rapidly begin to reduce its weight and function from May, after the surface water temperature reached to about 19°C.Almost all males examined here enough ripen in natural condition, but females do not, bearing rather mature eggs developed primary or tertiary yolk stage in which the largest eggs grow up to 0.43 mm to 0.77 mm in diameter.In addition, there is a tendency that the ovaries are much heavier in wild fish than in cultured fish (Fig.4).
日本魚類学会の論文