You are able to tame it off of berries and/or meat but the most effective way in my opinion and probably everyone else's is to lure a microbe swarm to the turtle, the turtle will eat the microbe fish or the microbe fish eat something from the turtle, anyways you need to stay near by and protect both the fish and the turtle from salmon, sharks and eels. I hope this helps.
They have patched the render range range so ignore any tips that say to leave render as it will no longer. Guess I’m stuck with the turtles I have, I’ve spent three hours trying to take one of these things and had it walk out of the water which reset it taming! It was at 80% that time. Never got it above 20 after that. What he for the saber tooth salmon that spawn to kill the swarm if they bite the turtle it resets to zero and you start over. Extremely frustrating way to tame a creature. I would like to beat whoever came up with this idea.