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.
Distribution and migration of the cyprinid fish Hemibarbus barbus in the Chikugo River were studied during 1986 and 1989. Many mature males and females were caught at the middle reaches in April, ...whereas most of the subadult fish were caught in the tidal reaches all the year round. According to monthly changes of gonadosomatic index of adult fish, it is clear that this fish spawns mainly in April. In order to elucidate the spawning run, 195 mature fish were marked at the tidal reaches during February and March, 1988, and 4 specimens were recaptured at the middle reaches in early April, 1988. In the tidal reaches, most of the fish were caught in waters less than 0.2 percent in salinity. Yearling fish were recognized in the lower reaches in May and began to be collected in the tidal reaches in July. These observations indicate that the yearling migrates from the lower reaches to the fresh water area of the tidal reaches. After staying there for 3-4 years, mature males and females run up for 20-40km for spawning in spring. Migration occurring wholly within fresh waters is called potamodromous migration. It seems that the migration of H. barbus in the Chikugo River is potamodromous migration and classified as an anadromous migration type.
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.
The embryonic development of Plecoglossus altivelis altivelis was described on the basis of eggs inseminated artificially on October 24, 1984 in Lake Ikeda. The eggs were spherical, 0.87-0.92mm in ...diameter. The incubationp eriod was about 230 hours at a water tempera-ture of 14.5-23.2°C (av. 20.1°C). The standard length newly hatched larvae had a small yolk sac (av. 5.3mm) and 61-62 myotomes. Within several days of hatching, the larvae attained 6.0-7.9mm in standard length and had completely consumed their yolk substances. Morphological changes of the larvae accompanying their growth were observed on 1.263 specimens collected by the larval net (80cm in diameter, 0.353 in meshes), the surf zone net (1m×5m, 1mm) and the cast net (13mm). The larval fin fold gradually disappeared, and the rudimental dorsal, the anal, the ventral and the adipose fins were visible at 11.4, 12.8, 24.3, 44.5mm in standard length, respectively. The conical teeth appeared at 24.3mm and the comb-like teeth at 55mm. Larvae smaller than 25mm inhabited the offshore waters of the lake, while those larger than 27mm appeared in the surf zone of the lake shore. The vertebral centrums of the larvae ossified at a size of 27.3mm. It is suggested that the considerable morphological changes of the ayu occurred at sizes of 25-30mm and 50-55mm in standard length. The former corresponds to the end of the larval stage and the latter to the end of the juvenile stage, respectively.
Observations on the life history of land-locked Ayu Plecoglossus altivelis were made in Lake Ikeda, in Kagoshima Pref., from 1981 to 1985. In this study, two types of Ayu were recognized in its ...social behavior of immature adult fish in the Lake. One is territorial type and the other is non-territorial type. Some differences were found in the shape of dorsal fin and the color pattern of body between two social types. The territorial fish have longer posterior dorsal fin rays and darker fin membranes than those of non-territorial ones. The former have three clear yellow oval marks on their gill covers and anterior body sides, however, the latter have only one mark and its coloration is pale or faint. Thesevariations are not related with body size and sexuality but with the social type. It is suggested that large and clear yellow marks are signal for recognition of their status among owners of territory and for showing the domination of owner to non-territorial fish. Thus, these differences between two social types of Ayu show a possibility of being applied to the evaluation of fishing ground of this species.
Spawning behavior of the landlocked ayu Plecoglossus altivelis altivelis was observed at five spawning grounds in Lake Ikeda. The spawning season of the ayu in the lake was from early October to late ...November, when the water temperature falled 18-23°C. Ayu spawned mainly at shore line shallower than about 30cm in depth and the substrate of redds were consisted of fine gravel (φ2-150mm). The number of spawned eggs, wind velocity and direction, and wave size at each spawning ground were measured daily from late September to early November. In most cases, spawning behavior appeared to commence with crosswinds. Spawning behavior was most frequently observed at a wind velocity of 4-5m/sec. These results suggest that crosswinds result in waves which trigger the onset of spawning behavior in landlocked ayu in Lake Ikeda.