“It's no secret in any way,” the zoo's chief executive said.

A Swedish zoo has admitted to killing nine healthy lion cubs since 2012.

The animals at the zoo, Boras Djurpark, were rejected by their pride and the zoo was unable to re-house them, The Independent reported.

“At that time we had tried to sell or relocate them to other zoos for a long time but unfortunately there were no zoos that could receive them, and when the aggressions became too big in the group we had to remove some animals,” Bo Kjellson, chief executive of the zoo, told Swedish broadcaster SVT.

Lions are listed as a “vulnerable” population on the International Union for Conservation of Nature's "red list" of endangered animals.

Kjellson says that the euthanasia was necessary to his organization's aim of conservation, as they wanted the healthiest population to be reproducing. The animals killed, however, were healthy, and Kjellson says they were killed as a matter of balancing the breeding program.

“It's no secret in any way and we do not try to hide that were working this way. So it's unfortunately a natural path for groups of lions,” he said. “To kill animals as part of the organization, I think that upsets quite a few.”

Zoos across the world use euthanasia as a tool to manage their animal populations, and the practice is permitted in Europe. Approximately 3,000-5,000 animals in European zoos are euthanized for this purpose each year, the executive director of the European Association of Zoos and Aquariaums told the BBC in 2014.