National Research Institute of Fisheries Science, Fisheries Research Agency, Ueda, Nagano 386-0031, Japan
The species composition, population density, growth rate and food content of fishes in the Urano River, a tributary of the Chikuma River, were investigated. A total of 7,052 fish belonging to 14 species were captured. The pale chub Zacco platypus, an invasive species from western Japan, was most dominant and accounted for more than 40%, followed by Misgurnus anguillicaudatus, Tribolodon hakonensis and Pseudogobio esocinus esocinus. Zacco platypus were abundant in running waters, whereas M. anguillicaudatus and P. esocinus esocinus mainly used pool areas. These dominant fishes grew positively from June to July or August. Most fishes preyed on aquatic insects such as chironomid larvae and ephemeropteran nymphs and only Z. platypus mainly fed on benthic algae. The piscivorous niche in the Urano River is almost vacant due to the extinction of native Japanese fishes such as the Far Eastern catfish Silurus asotus. Therefore, the Urano River is threatened by the invasion of piscivorous alien fishes such as largemouth bass Micropterus salmoides and smallmouth bass M. dolomieu.