Get a tamed dino and the Ichty will swim to it.
Spam pillars around it so it wont run away. 9 pillers should be enough.
Ehen tamed NEVER EVER Leah it in passiv im the ocean or in a river.
Pyrania will kill it.
The first time I saw one of these, I was on a rock in the middle of the sea taming a Pteradon. It came close, scared me, and since I thought it was aggressive, I gritted my teeth and bravely killed it. Shut up, noob's mistake.