http://en.wiktionary.org/wiki/muir

Rough translation - 'Sea Loch', which is where all salmon farms are located, rather clever. But I do feel a bit cheated to find out that M&S don't have their own loch. I guess the clue was the TM after Lochmuir.
http://www.marksandspencer.com/Pre-Slice...B00194DHK6

I did a bit more checking and found this:
http://www.scottishseafarms.com/en/our-s.../loch.html
So it turns out that M&S do have dedicated farms for their fish to ensure the quality. Roughly what I had hoped for even if Lochmuir ™ does not really exist.

This is better than when they claimed all their underpants had been personally stitched by St Michael.