Dynamics of gonadosomatic index of fish with indeterminate fecundity between subsequent egg batches: application to Japanese anchovy Engraulis japonicus under captive conditions |
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Authors: | Michio Yoneda Hajime Kitano Sethu Selvaraj Michiya Matsuyama Akio Shimizu |
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Institution: | 1. National Research Institute of Fisheries and Environment of Inland Sea, Hakatajima Station, Hakata-Kinoura, Imabari, 794-2305, Japan 2. Fisheries Research Institute of Karatsu, Department of Joint Research, Faculty of Agriculture, Kyushu University, Karatsu, Saga, 847-0132, Japan 3. Laboratory of Marine Biology, Department of Bioresource Sciences, Faculty of Agriculture, Kyushu University, Hakozaki, Fukuoka, 812-8581, Japan 4. National Research Institute of Fisheries Science, Fukuura, Kanazawa, Yokohama, 236-8648, Japan
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Abstract: | The gonadosomatic index (GSI) is widely used as a simple measure of reproductive capacity, but its validity has often been questioned. This study showed the inter-spawning variation in the predicted GSI of Japanese anchovy using the gravimetric oocyte packing density method. Time course sampling showed that final oocyte maturation and subsequent ovulation occurred from afternoon to evening on the day of spawning. Oocyte in vitro assay, however, suggested that the timing of the spawning could be predetermined by midnight the day before spawning. Models predicted that the GSI of a female just after spawning gradually increases until the completion of germinal vesicle breakdown, after which it dramatically rises by 24–53 % per hour within a few hours of the onset of ovulation. This indicates that the predicted GSI of a female increases by 3.5 times within about 19 h before ovulation, even if the relative batch fecundity remains constant. |
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