池田湖における陸封アユの成熟と産卵

抄録

Observations on the life history of the land locked ayu Plecoglossus altivelis altivelis were made in Lake Ikeda, Kagoshima Prefecture, 1981 to 1986.
The gonads of males and females developed rapidly from August and the GSI of both sexes indicated the highest values in October. The diameter of ovulated oocytes in the body cavity ranged from 0.74 to 0.99mm and the relation between the number and standard length of fish was represented as E=0.00188χL3.418.
The spawning season of the ayu in the lake was from early October to late November. The ayu spawned mainly at shore lines shallower than about 30cm in depth and the substrate of reds consisted of fine gravel, ranging mostly from 3-10mm in diameter. The sex composition of the ayu crowding the spawning site was not the same throughout the spawning season. Males predominated in the early period of the season, and females in the latter.
The male fish matured simultaneously and most of them crowded at spawning grounds. On the other hand, the fully matured female appeared at the spawning site one by one, while the immature ones dispersed around the site in low density. It is suggested that this mode of spawning in the ayu may be one of the important factors for sustaining its population land locked in Lake Ikeda.