Reproductive characteristics of the Ryukyu-ayu, Plecoglossus altivelis ryukyuensis

奄美大岛和冲绳的香鱼种群被描述为一个亚种（Plecoglossus altivelis ryukyuensis）。然而，很少有人知道这个亚种的生殖特征。近年来，为了恢复曾经在冲绳消失的香鱼，人们进行了幼鱼放流到河中的养殖生产，在养殖生产过程中，经常在养殖的雌性中观察到过熟的卵巢卵。在本研究中，在6月将常温条件下饲养的树蛙转移到长光周期（18L6D）条件下。在此条件下，雌性在夏季和秋季仍被抑制，而在环境条件下的对照组在11月下旬达到成熟。恢复到自然光长条件下，约一个月后，性腺成熟迅速，产卵受精率高于对照组，表明亲鱼产卵季节早于本地鱼种。研究了限饲引起的生长延迟的影响。试验分为正常饲料（CF）和低水平饲料（LF）两组。在6月20日实验开始后，LF组的体长和GSI在整个实验期间均显著低于CF组。大小的差异反映了卵黄发生和成熟开始的差异。CF组在11月下旬至12月上旬期间观察到排卵，其中42只雌性具有过熟卵。LF组在12月中旬才开始排卵，而LF组只有1头过熟雌鱼相伴，这表明香鱼的生长是影响本地河流产卵期开始的重要因素。

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.