In South Australia, a number of populations have been established through reintroduction of captive-bred animals.

Still, wildlife experts said, those captive-bred animals are, by their nature, wild, and owning them carries risks.

"Reintroduction of captive-bred animals is the tool of choice in only a few cases," he said.

Multigenerational captive-bred animals are neither domestic nor wild, these critics maintain.

But he raises continuing problems involving specifying the size of enclosures and the distinction between "free-roaming" and "captive-bred" animals.

And to make sure that the colony has a chance of getting established, the intention is to release a second, larger group of captive-bred animals next autumn.

Efforts are underway to reintroduce captive-bred animals to suitable wild habitats, but thus far have not met with much success.

But some biologists have criticized such reintroduction programs, saying captive-bred animals are sometimes easy prey because they do not know how to behave in the wild.

The program distributed to circus spectators argues that captive-bred animals help preserve species in the wild.

The disorder is also regularly found in captive-bred animals fed on high fish diets, such as mink, pigs and poultry.