There is no justification for the enslavement of animals.
None. There never has been and there never can be. Slavery is slavery-unjustifiable, cruel, evil and soul-destroying, both for the victims and the oppressors.
Circuses and places like SeaWorld are despicable places, and the only zoos that should exist are those that rehabilitate wildlife, contribute to the conservation of habitat, and provide medical facilities for animals in the wild.
Up until 1979, adult mountain gorillas were murdered so that baby mountain gorillas could be captured and displayed at the Cologne Zoo. This was once a routine practice by zoos, and today this method continues with the mass slaughter of dolphins in drive hunts designed to capture dolphins for trade and display in dolphinariums around the world.
(Photo: Sea Shepherd)