琵琶湖産ワタカの生活史

Abstract

Ischikauia steenackeri (SAUAGE) seems to be an endemic cyprinid, abounds in Lake Biwa. It inhabits almost all over the lake, but mainly prefers shallow and weedy parts of southern or eastern coast of it. It swims chiefly near the surface of water, and feed principally on water weeds in adult. In the breeding season, the males have minute, white and conical pearl organs appearing on dorsal part of head and body, around eyes, preopercles, lower jaw and inside of pectoral rays. The females have also them, but very weak and entirely on pectorals. From June to July, they breed on aquatic plants or root of willow tree. The spawning act is usually continued from twilight to about 9 o'clock at night, but also said to do in daytime when the water is swollen caused by heavy rain. The breeding behavior is generally similar to that found in carp or goldfish. The number of ovarian gees runs from about 2, 500 to 140, 000. It increases in accordance with increment of the total length and body-weight as is the case with Gnathopogon elongatus caerulescens (SAUVAGE) as formerly revealed by the present author (1949). The increasing trend is paraboric in the case referred to the total length and linear in the other case. The egg is adhessive, spherical in shape and 1.24-1.31mm in diameter. The egg membrane thin and transparent. The yolk is bright yellowish green and granular with a few small oil globules. The egg fertilized hatches about 3 days in the water ranging 22-24°C in temperature. Newly hatched larva measures 4.6mm in total length. The larva in 24 hours after hatching attains 5.0mm with numerous filamentous tubercles on laterals of the body as seen in G. elongatus caerulescens (SAUVAGE) (Nakamura 1949). These tubercles are also present in head when the fish grows to 6.0mm in 4 days after incubation. The larvae are found completely absorbed their yold-sac after 6 days from the hatching, and feed on small zooplanktons such as Rotifera and Cladocera. Judging from the scale total length relationship, the fish seems to grow about 50-100mm in Jst summer, 150-200mm in 2nd one and becomes mature, and then it grows very slowly.