| Budget Amount *help |
¥2,000,000 (Direct Cost: ¥2,000,000)
Fiscal Year 1995: ¥700,000 (Direct Cost: ¥700,000)
Fiscal Year 1994: ¥1,300,000 (Direct Cost: ¥1,300,000)
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| Research Abstract |
The ayu population in Amami-oshima and Okinawa Island was described as a subspecies (Plecoglossus altivelis ryukyuensis). Little is, however, known about the reproductive characteristics of this subspecies. Recently, aquaculture production of young fish has been conducted to be released into the river, and to restore the ayu which once disappeared in Okinawa Island.
In the process of aquaculture production, overripe ovarian eggs were often observed in cultured females. In this study, the syu reared under ambient condition were transferred to long photoperiod (18L6D) condition in June. Uuder this condition, females remained suppressed in summer and autumn, while control group under ambient condition attained maturity in the late November. When they were returned to the condition of natural day-length, gonadal maturation proceeded rapidly and resulted in spawning eggs with high fertilizability compared with that of control group after about a month.
It is known that the spawning season of broodstock is earlier than that of native stock in the river. The effect of delay of growth, which is caused by feed restriction, was investigated. The treatments were normal feed (CF,standard feed)and low feed (LF,feed at 1/3 of CF). After the commencement of experiment at June 20, body length and GSI were significantly lower in the LF group than that in the CF group throughout the experimental period. Size differences reflected differences in start of vitellogenesis and maturation. Ovulation was observed during the period from the late November to the early December in the CF group in which 42 females had overripe eggs. On the contrary, ovulation occurred in the middle of December in LF group in accompanying only one overriped female.
Based on these results, it is suggested that the growth of the ayu is important factor affecting the start of spawning season in the native river.
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