Hahaha - the resident expert.....200 miles from the beach with a glass of Highland Park in hand. Anyway to serious business....my thoughts are well known I think....if codling stay in the weirs long enough they go the colour of the afore said. That happens in the summer when the feed for those fish are in the weirs - they become resident fish - they are very local fish...for example every codling I have caught from St Abbs rock ends in the summer has been red. Every codling caught in the boat off St Abbs, a change of habitat of maybe 100 yards has been fresh run.
With regard to the winter I have never caught a red codling in the North East - the reason? - the feed is no longer exclusively in the weirs, there's worms being washed out along the sands (and up the skeers) so the fish are more mobile and less territorial. Are there any there? I suspect not but there may well be some in the deeper water further north. Might be worth one of our younger academics getting a bursary to pursue lol