Takahashi Rumiko Gekijou: Ningyo no Mori

According to an ancient legend, mermaid's flesh can grant immortality if eaten. 500 years ago, Yuta unknowingly ate a piece of mermaid's flesh. For centuries, he travels across Japan, hoping to find a mermaid, thinking she may be able to make him a normal human again. When he finally finds one, he discovers that she and her companions have been raising a girl to be their food so they can eat her and take on her youthful looks. That is how mermaids stay young. Yuta kills the mermaids and rescues her, but she has already eaten some of the mermaid's flesh. Although he had to kill the mermaids, Yuta isn't too disappointed. Yuta's once lonely existence is now over, as he has found a companion in Mana. And Mana, who had been trapped in a small hut her whole life, finds delight in even the simplest of things. Together, Yuta and Mana attempt to seek out more mermaids, trying to become normal humans again.

(Source: Anime News Network)