サワラの産卵期における卵巣内卵数と卵径組成の経時変化  [in Japanese] Changes in Ovarian Egg Number and Egg Diameter Composition in Spanish Mackerel Scomberomorus niphonius during the Spawning Season  [in Japanese]

To determine the spawning ecology of fish, changes in ovarian egg number and egg diameter composition during the spawning season were examined with material of spanish mackerel Scomberomorus niphonius. Gonadosomatic indices of female and male both fluctuated very similarly. The indices indicated peaks in early June, thereafter reduced rapidly, and tended to rise again in late June. The Relationship between fork length (L) and ovarian egg number (E) was expressed by a linear regression as follows:E=6.881•103•L-3.244•106 (γ=0.768) Thereafter, individuals whose egg number plotting points shifted below the initial regression line increased gradually with the lapse of time. Since considerable numbers of eggs still remained in the ovaries of individuals, they would continue to spawn. The composition of egg diameter showed plain polymodal forms at any sampling time. This indicated that some different egg diameter groups always existed at the same time. According to the results obtained, it was concluded that Scomberomorus niphonius spawns several times within a spawning season. This method of tracing egg number and egg diameter composition was considered to be useful to determine the spawning ecology of fish.