Fish spawning habitat preference was examined in an irrigation ditch of the Houki River in the Naka River system. Habitat types in the ditch were classified into five categories based on the physical characteristics of the environment. Eggs and larvae of Lefua echigonia and Cobitis sp. BIWAE type C, and eggs of Silurus asotus were collected in the ditch. Eggs of Lefua echigonia and Cobitis sp. BIWAE type C were observed in narrow, slow flowing, shallow areas out of the main current near the edge of the ditch in plant litter or vegetation. However, larvae of L. echigonia were found in wider and more vegetated areas in the main current than eggs. Eggs of S. asotus were found in wide, deep, slow moving areas in which litter and vegetation had been deposited. The findings of this study illustrated that the importance of microhabitat in spawning site selection, as well as the need to consider these factors when designing conservation measures for these species.

View full abstract

Population genetic structure of a common freshwater goby Rhinogobius fluviatilis in the Yoshino River and its tributaries was surveyed using three microsatellite loci to examine effects of dams. Allelic richness of the Akui and Sadamitsu River populations below the dams were high (15. 7, 15. 9) and that of the above the dams populations were low (10. 9～13. 0). Heterozygosity was same level diversity between above and below dam populations (0. 870～0. 890) except for Takeno river population (0. 788). Considerable genetic differences were observed between populations above and below the dams. Migratory history was also examined using otolith Sr/Ca ratio. The otolith Sr/Ca ratio analysis confirmed that the populations above the dams were landlocked by the dams. It was also suggested that some individuals below the dams do not undertake amphidromous migration.

View full abstract

In order to apply acoustic telemetry for tracking catfish, we first examined the effects of transmitter implantation on health of catfish in indoor water tanks using six catfish. As a result, the 5 weeks observation showed that there was no falling off of the dummy transmitter and death of fish, while the body weight of test individuals decreased slightly. Next, for better understanding of the behavioral activity of catfish，the tracking survey was conducted in a small stream named the Yagawa River in Tochigi Prefecture. Five catfish caught in the stream were tagged with the acoustic transmitteres and released， and their behavior were monitored in the detection range of the receivers. Three of the five acoustically tagged catfish were successfully monitored for more than 100 days in the study area. For a long time，these individuals have been settled in the same place where the log mattress bank was constructed. This result indicates that catfish has strong site fidelity and favor burrow habitats such as log mattress. The hourly signals of two fish were detected at nighttime more than that in the daytime from October to November. These signal detection patterns in autumn suggest that they stayed inside burrows in the daytime and moved outside their burrows at nighttime，which is considered to be typical nocturnal activity patterns of catfish. From December to January, however, hourly signals of three fish were detected all day and detection patterns were arrhythmic. These signal detection patterns in winter imply that catfish were active all day. Thus, this study quantified long term nocturnal activity patterns of catfish and for the first time found seasonal changes of these patterns. These new findings about behavioral activities of catfish provide information necessary for its conservation considering the catfish's life history in a river.

View full abstract

[in Japanese]

View full abstract

We reconstructed the distribution of aquatic plant community, including submerged plant, in Lake Shinji using aerial photographs taken by the US forces in October 1947. We identified the submerged plant community with the following features: shade and darker than sand and open water; profile, round in shape and existing below the water surface. We found only submerged plant community in the lake. Although Phragmites australis has been widely planted as the restoration project at Lake Shinji, emergent plant community was found only near the river mouths. Floating-leaved plant community or submerge plant community with extended leaves, that protrude from the water surface, were not found in the lake. The area of the submerged plant community was calculated as ca. 3km². Transparency had been formerly estimated to be 3m, but our results suggest that it was more than 4m in 1947. At present, the lake bottom at a depth of 4m consists mostly of mud. On the other hand, data from 1947 shows that the lake bottom was composed partly of sand, based on its reflection of white light and the existence of a ripple at the bottom. The deepest areas covered with submerged plant community were found at the center of the north shore of Lake Shinji, and its depth was estimated about 4m. Recently, submerged plants with extended leaves, that protrude from the water surface, have invaded Lake Shinji. Our results show that such type of submerged plants are not the same species or the same type of those found in the past which dominated the area.

View full abstract