Turns out that they did not measure it accurately. One of the scientists at Pitlochry estimated it at 45lb plus, they were unable to give a more accurate figure than that. The fish had been in freshwater for up to 5 months, so would have certainly been well over 50lb when fresh. There were no scales taken, salmon start to reabsorb their scales after a long period in fresh water and it is extremely difficult to obtain scale samples from really coloured fish, especially cock salmon whose skin goes almost leather like.