「かいぼり」で水質改善 Draining of water to improve water quality
昔の生態系が戻る Restoration of the original ecosystem
井の頭公園（東京都）では2014年1月、池の水を抜き、約1か月間、池の底を天日干しにした後、水を戻す「かいぼり」が行われた。それから約1年。水はきれいになり、外来種を取り除いたことで元の生態系が戻りつつある。
In January 2014, a practice known as "kaibori" took place in the Inogashira Park (Tokyo). Ponds in the park were drained of water to dry up the bottom in the sun, and after approximately one month, the water was poured back in. It has been about a year since, and now the water in these ponds is clean, and with the removal of foreign fish, the original ecosystem is gradually returning.
井の頭公園には弁天池、お茶の水池、ボート池、ひょうたん池の四つの池があり、かいぼりは弁天池以外の三つの池で行った。水質を改善することと外来種を取り除くことが目的だ。
The park has four ponds, which are Benten Pond, Ocyanomizu Pond, Boat Pond, and Hyotan Pond, and water was drained from all ponds other than Benten Pond. The aim is to improve the quality of water and to remove foreign fish.
水の透視度はかいぼりの前と比べて非常に良くなった。公園の管理事務所によると、かいぼりから1年たった2015年1月の水の透視度は、かいぼりをしたお茶の水池とボート池では100センチ以上だったが、弁天池は30センチだった。
Transparency of the water improved significantly compared to the state of the ponds before the draining. According to the Park Management Office, the water transparency level in January 2015, which is one year after the draining, was at least 100 cm in Ocyanomizu Pond and Boat Pond where the water was drained, whereas in Benten Pond, the level was 30 cm.
天日干しで、水質を悪化させる植物プランクトンが増えなくなった。そのため、日光が池の底まで届くようになり、泥の下に埋もれていた水草の種が発芽して育った。水草を食べてしまうソウギョを取り除いたことも水質が改善した理由の一つだ。
The draining put an end to the increase of phytoplankton, which deteriorates water quality. As a result, now the sunlight is able to reach to the bottom of the ponds, allowing seeds of waterweed, which were buried underneath the mud, to germinate and grow. Another contributing factor for improvement of water quality is the removal of grass carp, which feeds on waterweed.
生態系も変わった。ブルーギル（約1万2700匹）やブラックバス（約1100匹）などの外来種が取り除かれたことで、これらの餌となっていた在来種の魚、モツゴやナマズなどが増えた。モツゴを餌とする水鳥のカイツブリも池に戻ってきた。
Change also occurred to the ecosystem. Removal of foreign fish such as bluegill (approximately 12,700 fish) and black bass (approximately 1,100 fish) led to increase of the domestic varieties such as stone moroko and catfish. Grebe, a water bird that eats stone moroko, also returned to the ponds.
かいぼりはあと2回行うが、それだけで、昔のような「そこが見えるほど澄んでいた」池が完全に取り戻せるわけではない。
While kaibori is planned to be carried out two more times, it will not bring back the state of the ponds which used to be "so clear that the bottom could be seen".
井の頭公園の池は、昔は豊富だった湧き水によって、自然に池の水が入れ替わっていた。しかし、公園の周りの田畑に建物が立ち、道路がアスファルトで覆われると、湧き水は減っていった。現在は、1日約3800トンくみ上げる井戸水を入れ替え用に使っている。
In the old days, the water of ponds in the Park used to be naturally regenerated because of abundant spring water. However, as buildings were constructed on fields around the Park and roads were paved with asphalt, the amount of spring water began to decrease. Today, approximately 3,800 tons of well water is pumped out each day to replace the water of ponds.
2015年2月、井の頭公園のかいのぼりに関わる人たちの会合が開かれた。主なテーマは「どうやって市民に関心を持ち続けてもらい、協力の輪を広げるのか」だ。メンバーは「学校の課外授業やボランティアなど、楽しみながら関わりを持っていくことが池を大切にしていくことにつながる」と話す。
In February 2015, a meeting was held among those who are involved in the draining of ponds in the Inogashira Park. The main theme was on "how to keep up the interests of citizens and to promote their cooperation". The members agree that "as people enjoy their involvement in the project, whether as extracurricular activities at schools or volunteer work, they begin to feel inclined to take care of the ponds".
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