This question has changed. It use to be about killing a koi fish- for which the answer is below. Now it is about “fishing” for koi. So the answer is YES in some areas it is illegal. read more
Killing a koi in Japan is not illegal - if it were, every koi producer would be in jail. Koi are regularly culled continuously. Undesirables are recycled into feeds and other products. Personally I had a koi over there being raised by the farmer - it ended up with a large tumor so it was killed. It happens. read more