池田湖産陸封アユ仔・稚魚の成長に伴う分布と食性の変化

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Some observations on the life history of the landlocked ayu Plecoglossus altivelis altivelis were made in Lake Ikeda (4km in diameter, 233m in depth) in Kagoshima Pref. from 1981 to 1986. A total of about 1, 263 specimens were collected with a larva net (80cm in diameter, GG50), a surf zone net (1×5m, 1mm mesh) and a cast net (24 meshes). The larva net was towed for 10 minutes on the surface and at a mid-layer at 2-3 knots in the daytime and after sunset. Newly hatched larvae were 4.9-5.6mm in standard length, scattered in the offshore waters of the lake. They were distributed amongst 3-24m depth layers, mainly in the 7m layer, in the daytime and appeared at the surface soon after sunset. Although individuals smaller than 25mm inhabited the offling of the lake, ones larger than about 30mm occurred at the surf zone of the shore. The larvae of the ayu began to feed on zooplankton such as Protozoa and Trochelminthes before absorption of the yolk substance, and to eat mainly active zooplankton belonging to Arthropoda as they grew. Juveniles smaller than 45mm fed on zooplankton and other animals, but individuals larger than 55mm switched their diet to algal food such as adherent diatoms and blue-green algae. It is suggested that the standard length at the prelarval, postlarval, juvenile and young stages of this fish in Lake Ikeda are 5-8, 8-25, 27-50 and over 55mm, respectively.
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