Can fishermen really move once every turn? Does that mean during a round they could circle the loch several times if they wanted?
From the rules:
Once per turn, you can move a Fisherman to an adjacent loch space by rowing but only before your main Action.
This means that the McDonald player can move a SINGLE Fisherman during his/her turn.
If you really wanted to move a single fisherman around the Loch this way, you're welcome to, but eventually they'll want to stay put to establish a settlement and for settlement scoring.