Biodiversity / Food / Water

October 22, 2012


Improved Water Quality Sees Migration of More than 10 Million Sweetfish

Keywords: Ecosystems / Biodiversity Local government 

The total number of sweetfish (or ayu in Japanese) that has returned to Tama River for spawning in 2012 is estimated to reach 11.94 million, the highest total since the survey began, the Bureau of Industrial and Labor Affairs of the Tokyo Metropolitan Government announced on June 20. The survey to count ayu migrating upstream to spawn was carried out for 70 days from March to May 2012, using fixed nets at the mouth of the Tama River, by the Tokyo Metropolitan Islands Area Research and Development Center of Agriculture, Forestry and Fisheries.

In the Tama River, the number of upstream-migrating ayu was estimated at 180,000 in 1983 when the fish count began. The numbers then fluctuated but remained below one million over the following 20 years. From 2006 to 2011, the estimated migratory population exceeded one million for five consecutive years, and in 2011 it increased drastically to 7.83 million, the 4.2 times up from the previous year. In 2012, the number has reached 1.5 times that of 2011, showing a remarkable increase from one million to ten million in only two years

The recent increase in the migratory population is thought to be the result of better water quality brought about by improvements in sewage treatment along the river. The improved water quality in Tokyo Bay, as well as suitable spawning sites being rebuilt following a typhoon in the autumn of 2011 using sand dredged from river bottoms, are also among factors contributing to the migration.

Related JFS article:
57 Percent of Rivers across Japan Clean, Aquatic Life Survey Indicates
Run-up of Sweetfish in Tama River Reaches Record High of 1.96 Million
How "Fresh" Is the Water in Japan's Major Rivers?