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Abstract

English Abstract

The responses of fish populations to experimental habitat improvement in a concrete-lined, altered reach were examined in a fourth-order tributary of the Teshio River, northern Hokkaido, Japan. Concrete blocks in the center of the channel were removed, and log-drop structures were installed across the stream to enhance the fish habitat which had been degraded by the channel alteration. Subsequent changes in habitat variables and characteristics of the fish assemblage were studied in the altered treatment sections, which were compared with adjacent tural, control sections in both the pre- (1993) and post-improvement periods (1994). Before improvement, Oncorhynchus masou, Tribolodon ezoe and Cottus nozawae in the tural sections were considerably more abundant than those in the altered sections having shallower depth, greater current velocity and smaller substrate coarseness than the former. In contrast, the abundance of Noemacheilus toni in the altered sections did not differ from that in the tural ones. The habitat improvement caused marked changes in habitat variables, including increased depth, decreased current velocity and increased substrate coarseness, with development of pool-riffle sequences. After the improvement, significant increases were seen in the abundance of O. masou and T. ezoe, but not in that of C. nozawae. The improvement caused an increase in the body size of N. toni, but not in its abundance. Consequently, this fish population response resulted in a great increase of species diversity of the assemblage during a relatively short period (1 year) after the habitat improvement.