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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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.
KeywordsGonadosomatic Index Vitellogenic Oocyte Ovarian Maturation Japanese Anchovy Oocyte Size
We are grateful to the staff and students of the Fishery Research Laboratory and Laboratory of Marine Biology at Kyushu University for their support in rearing and measuring the specimens. We also thank three anonymous reviewers for valuable comments. This research was funded by a subproject on studies on the prediction and application of fish species alternation (SUPRFISH), financed by the Agriculture, Forestry and Fisheries Research Council of Japan, as part of the Population Outbreak of Marine Life (POMAL) Project.
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