The term "management euthanasia" is used when a healthy animal is culled for management reasons within a captive facility. It's been estimated that 3000-5000 animals may be management euthanised each year in Europe, although most of these are smaller, less "interesting" mammals, such as mice and rats. But more on that later.
Zoos and aquariums often euthanise otherwise healthy "surplus" animals because of overbreeding. Animals in zoos are bred for education, conservation purposes, entertainment or even for fundraising - cute young animals can guarantee increased footfall in a zoo. However, zoos and aquariums will often justify the culling of these surplus animals as being in the best interests of that breeding animal. Put simply, breeding is a natural behaviour, and therefore it must be positive for an animal's welfare. Consequently, breeding from an animal will trump the potential need to cull the offspring.
While encouraging natural behaviours is important for a captive animal's welfare, a) assuming that all animals desire the same behaviours is limiting, and b) the negative welfare consequences on the individual animal's welfare left behind is still poorly understood. For too long zoos and aquariums have justified this breeding and culling cycle from the animal's welfare perspective, but it's time that this was challenged.