Abstract
Recently, large numbers of young ayu (Plecoglossus altivelis altivelis), one of the most commercially important freshwater fish in Japan, have been found at the lowest reaches in the Kamo River, migrated upstream from the Yodo River. However, the migration of ayu was thought to be hindered by channel fragmentation through a series of drop works. Therefore, we investigated their distribution and habitat use by field observations in 20-km reaches of the Kamo River. Although the water temperature condition was higher than the physiological limit of the fish in the lower reaches of the river, a considerable number of fish survived in the reaches. In addition, we found that they foraged at riffles in the main channel during the early morning, whereas they moved to spring-fed pools behind the sandy bar during the daytime. Furthermore, their stomach contents and nutritional conditions during the afternoon were lower than those in the morning and worse than those in the middle reaches, respectively. These results indicate that the ayu population in the lower reaches of the Kamo River can survive because of the spring-fed pools as a refuge from high water temperature. However, considering that the growth of the fish was less efficient than that of those migrated upstream, the fragmentation of river channel seemed to undermine the production of ayu in the Kamo River. Therefore, improvement of the drop works by lowering the height and/or constructing fish ways as well as conservation of spring-fed pools and groundwater recharge is required for increasing the wild ayu population in the Kamo River.
