Kanagawa river yields three piranhas

They caught 3 piranhas there and have declared the stream to be off limits.   Piranhas aren't native to Japan.   What could have happened is that someone smuggled the fish in as pets, and maybe the pet owners decided that they do not want the fish and throw them into the rivers.

The few piranhas by itself aren't dangerous.   The great danger is when they adapt to the river conditions and start to multiple and proliferate to other water ways, that can make the entire nation's water ways dangerous.   I would appeal to anyone, if you have piranhas, surrender them to the authorities or if you do not want your piranhas, surrender them.  The huge dangers of throwing such fishes into the local streams cannot be overstated.

Some pictures of Piranhas taken from public domain pictures of Wikimedia.  They don't look as tame as these when they smell blood!   There are horror movies as well as real life horror attacks by piranhas.  Such fishes aren't suppose to be in your home aquarium at all.