池田湖における陸封アユの生活史に関する研究‐ＶＩＩ 池田湖産陸封アユの大型化について

Abstract

Observations on the life history of the landlocked ayu Plecoglossus altivelis altivelis were made in Lake Ikeda, Kagoshima Prefecture from 1981 to 1986. The body size of ayu in this lake was similar to that of Lake Biwa, but smaller than that of the amphidromous population in streams. The ayu grew during the early spring and mid-summer. The body sizes of mature fish were mainly 80-90mm in the present research. On the other hand, the mature specimens of ayu collected in 1953 and 1965 measured 56-60mm and 63-73mm in standard length, respectively. It is suggested that the size of mature ayu in the lake has been becoming larger over the past 30 years. The larvae of ayu, living offshore of the lake, fed on zooplankton, such as Protozoa, Trochelminthes and Arthropoda, but when they reached about 55mm in standard length, they moved to shore water and changed their food to algae, such as Cyanophyta and Bacillario-phyta which adhered to stones and rocks on the bottom. The amounts of these adherent algae in the lake have been increasing since 1929 according to the progress of eutrophication of the water. Thus, it is suggested that the increase in the size of mature ayu depends mainly on the good condition of food resulting from eutrophication.