Spirulina and the Lost Whale by Kelly McKain
Bored with sitting pretty on Mermaid Rock, Spirulina leaves her sisters and goes off to find someone else to play with. Whilst having fun with the fishes, she meets Splosh, a baby whale. But Spirulina's new friend has lost his mummy, and when they set out to search for her, all they find is a boatful of nasty fishermen and a large fishing net - just right for catching mermaids...!