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.

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