Treehouse Books & Melanie Voland 
The Soul Cages: Traditional Mermaid Folk Stories Collection [EPUB ebook] 

A kind, shrewd fisherman makes friends with an Irish merman, called a merrow,
And he visits his home, underneath the ocean.
But he soon discovers that the merrow is keeping something very strange, caged up there,
And he feels that he must do something about it…
This story contains subjects and themes that may not be suitable for younger children.