Significance of meandering channel morphology on the diversity and abundance of fish assemblages in a lowland river in Japan

Background/Question/Methods

When restoring aquatic ecosystems in an impaired river, the river landscape that was originally there provides reference features. Although channel units, such as pool and riffle, are often used as basic structural components of rivers, off-channel habitats within floodplains are rarely examined in combination with the channel units. Thus, studies aiming to understand both the main channel and floodplains as constituting the entire river landscape are very important. Our objectives were to elucidate structural units of the river landscape (river landscape units) in a meandering river and their functions as summer fish habitats compared to those in a channelized reach, and to demonstrate historical changes of the river landscape and fish community over the past 50 years in the Shibetsu River in northern Japan. River landscape units in the meandering and channelized reaches were classified based on hydrogeomorphic conditions. We electrofished each river landscape unit, and performed non-metric multi-dimensional scaling to examine the differences in fish assemblages among these units. We also investigated the distribution of river landscape units in a 2 km valley of the meandering and channelized reaches. Historical air photos of a 23 km valley were used to examine the historical changes of the river landscape.

Results/Conclusions

We classified the meandering landscape into ten units: bank edge, point bar edge, glide, backwater, side channel, oxbow and the first four units including wood. Side channels and the four units with wood were used by many fish of differing species. Oxbows provided a habitat particularly suitable for lentic fish. Each of the other units functioned as a habitat for different fish assemblages. The channelized landscape consisting of only four units (outside and inside of channel curve, straight units or straight unit with wood) provided very a limited number of habitats for fish. When agricultural and town developments occurred between 1947 and 1978, the composition of the river landscape units were greatly altered and fish were reduced by more than 50% in the studied 23 km area. The original meandering rivers that consisted of heterogeneous habitat units fostered various fish communities. Conversely, the current channelized reach has a limited variety of landscape units, resulting in the degradation of the gamma diversity in the Shibetsu River. It is of great importance to understand structural and functional changes in the river landscape as well as the differences between natural and modified rivers, which provide invaluable information to devise restoration strategies.