Dolphin is about 5130 HP right now to be more precise. It goes by Mvest rather than by HP. The number of HP per Mvest varies. Minnow is 1 Mvest, then you just multiply by 10 for each level after that. So dolphin is 10 Mvest, orca is 100 Mvest, and whale is 1000 Mvest. Right now it's about 513 HP per Mvest. If we check again tomorrow, it might be a different number.

>>> hive = Hive()
>>> hive.get_hive_per_mvest() * 10
5129.7050253851285