Over a 1000 dolphins spawning in a range of 100 blocks at a certain height

The bug

Over a 1000 dolphins spawning in a range of 100 blocks at a certain height.

How to reproduce

1. Generate a new world
2. Go to an ocean with dolphins
3. Look at the dolphins as there are just a few
4. Teleport or fly to about y=200
5. As the player is outside of the 128 despawn range all mobs despawn but dolphins keep spawning and not despawning
6. (You can see the fish spawn and flash out but the dolphins spawn and stay)
7. Type /kill @e[type=dolphin]
8. Be disgusted at the fact that you just killed 4000 dolphins
9. Rinse and repeat (just for fun)