Looking at black bass and bluegill caught fresh at Inokashira Park, Toshiaki Tanaka sighed with satisfaction at catching some of the nonnative species plaguing its picturesque pond. But at the same time, he said he was frustrated knowing that alien species remain firmly entrenched there despite the five years he and his friends have spent trying to fish them out.
“If nothing is done, this pond will be packed only with black bass and bluegill. . . . The balance of creatures in the park will be destroyed,” he said.
Tanaka, 59, heads the Inokashira Nature Watchers group, a nonprofit organization promoting biodiversity in the park, an oasis of cherry trees and fresh air straddling the cities of Mitaka and Musashino in Tokyo’s western suburbs.
Starting in 2007, with approval from the Tokyo Metropolitan Government, the group began capturing bluegill and black bass — freshwater species indigenous to North America — in the pond to protect its native species and ecosystem.
As of April 21, the group’s 23 members had caught a total of 289,101 bluegill and 24,309 bass. Their efforts, though low-key, have been effective in preventing the foreign fish from expanding their numbers — and saving native breeds from extinction.
Nevertheless, the group hasn’t been able to effect a major drop in the invaders’ population or reverse the decline in indigenous species.
“There are limits to what we can do,” Tanaka said. “The reproductive power of black bass and bluegills is high.”
Inokashira pond is just one of many rivers, lakes and other inland bodies of water suffering from growing numbers of foreign fish threatening native species that have existed for centuries.
Black bass initially entered Japan in 1925 via Lake Ashinoko in Kanagawa Prefecture, followed by the first bluegill in 1960, according to Environment Ministry data. The bass were apparently released by people who enjoy bass fishing, while the bluegill were released as bait, experts say.
The bluegill were reportedly brought from the U.S. to Japan as a gift in 1960 by Emperor Akihito when he was the Crown Prince, and given to a Fisheries Agency research institute for consideration as a food fish.
With their high reproductive power, the populations of the two fish exploded nationwide in the 1990s. By 2001, black bass had been spotted in all 47 prefectures and had established habitats everywhere except in Hokkaido, according to the ministry. Bluegill have also been confirmed in most prefectures.
To prevent further invasion, the Environment Ministry instituted the Invasive Alien Species Act in 2004 to ban the breeding, holding, transport and import of foreign species, including bass and bluegill. At present, the law covers 13 species.
Many other nonnative species spotted in Japan are spreading as unwanted pets released by disenchanted pet lovers. These include the guppy, a tiny tropical fish, and even vicious snapping turtles, both of which have been spotted in Inokashira Park.
Today, Inokashira pond is almost dominated by bluegill and bass. The native species, including the stone “moroko” (a type of carp) and the “toyoshinobori” rhinogobius, are in decline.
In the meantime, Tanaka said that the park’s “kaitsuburi” (dabchicks), which depend on the stone moroko as food, are nearly gone.
“A few years ago, there were three breeding pairs of dabchick (in the park). But now only one pair remains,” he said of the ducks.
Last fall, the Tokyo Metropolitan Government blocked off a corner of the pond to build a protected space where native fish species can live and hopefully breed. Tanaka said he is eager to see indigenous species flourish again but hasn’t been able to confirm an uptick in the population yet.
The metropolitan government also plans to drain the pond to clean the muddy water and to eliminate foreign species. A similar method was used a few times in the moat around the Imperial Palace and had been considered effective in eliminating the two interlopers.
However, Norio Iwami, a professor of environmental microbiology at Meisei University, said it’s anyone’s guess how long that measure will work.
“By draining the water, we can clearly see what kind of creatures are living in the pond and we can retrieve them. So it would be effective,” Iwami said. “But if an Adam and Eve remain, they breed again. If they can’t retrieve all the eggs, then I don’t know what will happen. We’ll have to wait for a year or two to know the result.”
Meanwhile, another conservative group has been taking different measures to protect indigenous species in the Tama River, which some have dubbed the “Tamazon” because of the many abandoned tropical fish spotted in it.
Mitsuaki Yamasaki, one of the leaders of Osakana Post, a nonprofit environmental organization based in Kawasaki, has been calling on people to dump unwanted fish in a tank it installed in the city instead of releasing them into the Tama.
Since the “osakana post” (fish post) was set up in Kawasaki in 2005, about 10,000 fish, including gar and axoloti, have been dumped into the tank each year, with many donated to schools or to the NPO’s registered members.
Yamasaki looks after about 1,000 of the fish at his home in Kawasaki. Although it costs him about ¥150,000 in electricity each month, He said his concerns for the river and his love of the fish keep him going.
“The Tama River used to be home to some 50 native species. But now some 250 species, including foreign ones, have been confirmed,” he said.
“What people don’t realize is that they live for a long time. For example, catfish live for 40 or 50 years and carp live for about 50 to 80 years,” he said. “People should think more seriously about whether they can look after such fish for such a long time before purchasing them.”
Educating people and discouraging them from dumping fish into the wild are the only ways the group can protect indigenous species, he said.
Thanks to the fish post and educational efforts, Yamasaki said almost no foreign species can be spotted around Kawasaki. But in other areas, things are about the same.
“Unlike the pond, we cannot drain water from a river. So the important thing is to stop people from dumping fish. Also, in the case of the highly reproductive black bass and bluegill, we have to capture them to some extent,” he said.
By doing that, native species may someday outnumber nonnative ones in the Tama — in 50 or 100 years, Yamasaki said.