With an ever-increasing number of foreign aquatic species threatening ecosystems in many areas of Japan, municipalities are calling on people not to release them into the local environment.

Most of the foreign species, such as bluegill and red-eared slider, are believed to have been released in ponds and other places by former owners. Local residents are attempting to eradicate them by means of “kaibori,” a traditional agricultural practice of draining ponds.

Japanese farmers used to conduct kaibori during agricultural off-seasons to prevent irrigation ponds from being silted up by mud. In recent years, kaibori has often been used for the purpose of environmental conservation as it is considered a practical way to catch foreign species.

In January, kaibori was conducted at a pond in Inokashira Park, in Kichijoji, western Tokyo. Of some 20,000 creatures that were captured through kaibori, two-thirds of them were foreign species such as black bass and bluegill, according to park managers. There was even a grass carp that can grow to a length of more than 1 meter, they said.

In addition to the foreign invaders, a total of 230 illegally dumped items — bicycles, minibikes and home electronics — were found. Some of them must have been thrown into the 45,000-sq.-meter pond by drunken merrymakers during cherry blossom-viewing parties, park officials said.

According to Inokashira Park, the water of the pond used to be so clear that people could see the bottom. Over time, the water quality has deteriorated due to urbanization in surrounding areas and depletion of spring water. Domestic species such as Japanese dace and brook lamprey have completely disappeared as the water became dirty and foreign species increased.

Two more kaibori sessions are scheduled to be carried out at Inokashira Park by fiscal 2017, when the park will mark its 100th anniversary.

Kazuhiko Daido, an official of Tokyo’s Seibu District Park Office that manages Inokashira Park, said the kaibori conducted in January was successful. “Together with local residents, we hope to continue efforts to bring back our original ecosystem,” Daido said.

In Nara Prefecture, the prefectural office drained the water of Sarusawa Pond in the city of Nara for the first time in 18 years in February. Some 200 red-eared sliders were captured — the number far exceeded that of domestic turtles in the pond, according to the prefecture.

But the prefecture did not kill the terrapins in consideration of the Buddhist observance called “hojo-e” that takes place in the pond, in which a local temple urges people not to conduct needless killing. Instead, the prefecture asked the Suma Aqualife Park in Kobe to keep the captured sliders.

Tomoko Nishibori, head of a citizens’ group that works to preserve Japanese turtles, said, “Kaibori is one of the useful means to exterminate foreign species. It should be conducted regularly to preserve Japan’s indigenous species.”

Last fall, the Environment Ministry launched an effort to eradicate foreign tropical fish in ponds near the Onneto Yu no Taki, a hot-spring waterfall near Lake Akan designated as a national natural treasure in Hokkaido.

The number of tropical fish such as guppy and tilapia started to increase at the ponds around 30 years ago. As hot-spring water runs into the ponds, the water is kept at an even 20 degrees even during the winter, which is a perfect environment for tropical fish.

To prevent warm water from pouring into the ponds, the ministry is planning to spend some ¥5 million on plumbing work. This will lower the water temperature and the tropical fish are expected to freeze and die.

Kunitaka Fujishige, an environment protection specialist at the natural environment office in the city of Kushiro, in eastern Hokkaido, said his mission is to bring the ecosystem back to its original state.

“I feel sorry for tropical fish that have to be killed,” he said. “We are urging people not to release foreign species.”

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