Euthanasia In Aquariums and Zoos

Perhaps it's not the most uplifting and positive topic of conversation, but there's a lot that goes into decisions about euthanasia in aquariums and zoos. What are we looking for in an animal to let us know that it may be time? How do we go about making such a decision? Who is involved? There are differing opinions, but there's a basic framework for this sort of decision... devastating as it may be.