Disappearing dolphins

So I was building a pool on the latest snapshot and I decided to add dolphins to it.At first they were able to leap out the pool but eventually I was able to keep them all inside with a higher wall.I came back the next day and they were gone so spawned more in and named them dolphin by naming the spawn egg.After I changed to peaceful they were gone.I red that neutral mobs are not sd to despawn if you switch to peaceful mode. I realised after typing this that they were gone before I switched to peaceful.I googled it and apparently other people have had the same problem.