Changes in the Distribution and Food with Growth of the Landlocked Ayu Larvae Plecoglossus altivelis altivelis of Lake Ikeda in Southern Kyushu

Abstract

Some observations on the life history of the landlocked ayu Plecoglossus altivelis altivelis were made in Lake Ikeda (4 km in diameter, 233 m in depth) in Kagoshima Pref. from 1981 to 1986. A total of about 1,263 specimens were collected with a larva net (80 cm in diameter, GG50), a surf zone net (lx5 m, 1 mm 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 4.9-5.6 mm in standard length, scattered in the offshore waters of the lake. They were distributed amongst 3–24 m depth layers, mainly in the 7 m layer, in the daytime and appeared at the surface soon after sunset. Although individuals smaller than 25 mm inhabited the offing of the lake, ones larger than about 30 mm 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 45 mm fed on zooplankton and other animals, but individuals larger than 55 mm 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 55 mm, respectively. © 1991, The Japanese Society of Fisheries Science. All rights reserved.