Pippy

In a world faced with a flood of overpopulation, the scientists of the world are called together to come up with solutions. Prof. Leeman suggests for man to live in the sea, but is shunned. In an act of desperation, he turns his son into a merman, and sends him into the sea. This boy is found by a mother sea turtle, and is given the name Pippy. Pippy, unaware of his true parentage, grows up alongside others in the ocean, until he has a terrible encounter with humans. Thus, Pippy goes into the human world, and a long journey awaits him.

Volume 137 of the Osamu Tezuka Complete Works