Manga by author Shimizu Reiko - Status ongoing

From ShoujoMagic:On Kabuchi-jima, an island ruled by Princess Kaguya, ten children were raised as sacrificial victims. Several of the children thought they had escaped their fate when they escaped the island. But nearly a decade later, the kids are still dying in the order and at the time they would have as sacrifices... Their only hope now is to reunite and return to the island to seek out and destroy the source--the legend, the blood-lusting Moon Princess--before time runs out.In 2002 Kaguya Hime received the 47th annual Shogakukan Manga Award for shoujo.

From CMX:Will a half-mermaid, half-human girl bring an end to the feud between humanity and the mer-people or bring about the destruction of all life on Earth? As a merman named Shona is returning to his birthplace to spawn, he meets a young human, Jimmy, who suffers from amnesia. Together, Jimmy and Shona set off to find the missing girl. Will they be able to avert a mysterious prophecy, or will finding the secret of Jimmy's identity delay them enough to set Earth on the path to destruction?