Tropic of the sea

For as long as their family tradition tells, Yosuke Yashiro's family has taken care of a mermaid egg once every sixty years. Once it hatches they return it to the sea, and in return the mermaids bless their seaside town with bountiful catches of fish and calm waters. However, Yosuke's father has gone astray, and now he is using the mermaid egg for financial gain as a tourist attraction. The mermaids withdraw their blessing, and if Yosuke doesn't return the egg his whole town will be destroyed.