Orkney Trout Fishing Association ensures that Bea Loch, North Loch and Roos loch are stocked with lovely freshwater trout.  No permit is required, but you should check with landowners if you need to cross a field to access the shore.  Bea Loch has a reputation for having some monsters that have been caught more than once, weighed and put back.  The more moderate ones make a tasty meal on bonfire embers.