A zoo in Japan has culled 57 snow monkeys by lethal injection after discovering that they carried “invasive alien” genes.

Officials at Takagoyama Nature Zoo in Futtsu city, Chiba Prefecture, conducted the cull after finding out that a third of its 164 resident monkeys were not pure Japanese macaques.

A total of 57 monkeys were found through DNA testing to have been crossbred with the rhesus macaque, which is designated an “invasive alien species” and banned from possession according to a Japanese law passed in 2013.

The monkeys were subsequently put to death by lethal injection over a period of one month before the zoo operator reportedly held a memorial service for them at a nearby Buddhist temple.