Survey Finds Improved Water Quality, More Non-Native Species in Rivers in City of Yokohama, Japan

Indoplanorbis exustus
Copyright Yokohama Environmental Science Research Institute

The Yokohama Environmental Science Research Institute, Environmental Planning Bureau of the City of Yokohama, Kanagawa Prefecture, announced on April 26, 2012, the results of an aquatic species survey in rivers running through the city. The survey has been conducted once every three years since 1973 on the aquatic life in six river systems in the city, including fish, benthic organisms, fresh-water plants, periphytic algae , and others. The aim of the survey is to assess the general water quality of rivers by examining the aquatic life.

According to the survey, an improvement of effluent water quality, thanks to the spread of the city's sewage systems, has caused the return of a variety of species, as seen in an increasing trend of species migrating between rivers and the sea, such as sweetfish and shrimp. Meanwhile, it was also noticed that the number of non-native species is on the rise.

Twenty-four non-native species from abroad were found, some of which inhabited more sampling points than in previous surveys. It appears that such foreign species are not only increasing in number but also in habitat distribution. For example, a small benthic snail (Indoplanorbis exustus ) is one of the species that was seen for the first time.

An assessment of water quality according to aquatic species at 41 sampling points found that 19 points were "very clean," 14 points were "clean," 3 points were "slightly polluted," 2 were "polluted," and 3 were "indeterminable." The level of water quality was roughly the same as that found in the previous survey in FY2008.

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