Mirabella Plunkett was a witch famous for falling in love with a merman in Loch Lomond while on holiday there. When her family forbade her to marry him, she transfigured herself into a haddock and was never seen again (FW).
Family forbade her to marry a merman with whom she had fallen in love (FW).
The most obvious conclusion from the given information is that Plunkett turns herself into a Haddock in order to live in Loch Lomond with the merman. However, the haddock is a saltwater fish and Loch Lomond is a freshwater lake. -BB