They are needed for taming megachelons. It's like a minigame. You will need to guide a fish school to the turtle, then kill the other fish (like Sabertooth salmon) that spawn to attack the parakeet school while you're taming the Megachelon.
There’s no way to get a dermis of one of these, hitting their corpse with a taxidermy tool does nothing, (I don’t even know if they have a corpse) and you can’t force tame them or summon them as tamed, they will be wild. (Don’t try to summon a high level tamed one, it’ll be wild and very hard to kill.)