Dredging and predation by Goosanders thought to be contributory factors in smolt deaths on way to sea.
Migratory routes suggest that smolts may have to pass the barrage of sea farm lice and pollution at Orkney & Shetland also:
https://www.deesidepiper.co.uk/news/concern-over-loss-of-young-dee-salmon-1-4870334