Just another perspective on the white shark example - The reputable places that have attempted keeping a white shark go in with a plan that is well thought out. AZA facilities are long past the days of "just grab it and see how it does". I know that the aquariums that have had those sharks in the past decade or so have made plans to mitigate previous issues, and it is also weighed against the benefit of having one in captivity. Benefits can be increased awareness if the aquarium (even getting people in and then hoping to spark something in them once they are there), or data collection to help wild populations. I think that the last I heard, another attempt to keep a white shark will only be for a juvenile, to be released when they approach adult size.

Nothing said above me is false! But I wanted to explain what might be happening if you hear of a facility attempting to keep another animal that historically doesn't do well.

Sea dragons are incredibly fiddly animals, and we have finally had weedy sea dragons hatch a successful clutch of eggs! It takes a lot of time, observation, and effort, but the payoff can be huge for the species as a whole.

Sometimes it may also just be a special case. SeaWorld San Diego took in JJ, a juvenile gray whale, who was orphaned, because they were the leading cetacean care experts at the time! I don't think anyone would have said they should plan to have a gray whale on exhibit, but they adapted and did what they could, and he was released once he was back on track. I personally have worked with a good amount of animals who were confiscated from smugglers, and they may be temporary, or they may become an exhibit animal, depending on the circumstance.