I have spent a lot of time building an aquarium and capturing sea life to populate it. Mostly this has been very successful. However I am really struggling to capture a dolphin. I have a large enclosure with high walls and deep water to allow them to jump/breath and not escape. I have named them with a name tag so that they will remain persistent.  However they still drown or despawn when I leave them. Is it possible to keep a persistent dolphin? Is this a bug? Is it just bad luck thus far? I just want to know if I should keep trying or if I should stop wasting my time. Thank you!