Reproductive characteristics of the Ryukyu-ayu, Plecoglossus altivelis ryukyuensis

北大岛和冲绳岛的阿育种群被描述为一个亚种(Plecoglossus Altivelis Ryukyuensis)。然而，人们对这一亚种的生殖特征知之甚少。最近，为了将冲绳岛上一度消失的鱼苗放流到河里，进行了水产养殖生产。在水产养殖生产过程中，经常发现养殖的雌性过熟的卵子。在本研究中，在环境条件下饲养的Syu在6月份转移到长光周期(18L6D)条件下。在此条件下，雌性在夏季和秋季仍然受到抑制，而对照组在11月下旬达到成熟。当它们恢复到自然日长的条件时，性腺发育迅速，大约一个月后，与对照组相比，产卵能力较高。众所周知，亲鱼的产卵季节早于河里的本地种群。研究了饲料限制对仔猪生长延迟的影响。处理为常规饲料(CF，标准饲料)和低饲料(LF，饲料为CF的1/3)。6月20日实验开始后，在整个实验期间，LF组的体长和GSI显著低于CF组。大小的差异反映了卵黄发生的开始和成熟的差异。CF组在11月下旬至12月上旬观察到排卵，其中42只雌性有过熟卵。反之，LF组的排卵发生在12月中旬，只有一尾被覆的雌鱼。根据这些结果，阿育的生长是影响当地河流产卵季节开始的重要因素。

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.