Haddock

Notes

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

• One possible explanation is that magic somehow allows a haddock to survive in freshwater - perhaps it's the same thing that allows Hogwarts's giant squid to live in the lake there.
• Another possibility is that Plunkett does disappear into the ocean. She was vacationing "near" Loch Lomond, which really isn't too terribly far from the coast (source: Google Maps).