In most salmonids, size is determined by age at maturity. This is particularly true of the semelparous (spawn once). Over the century plus of hatchery operations this has been shown time and again. But, as many here have noted, if you put intensive fisheries on immature fish the likelihood of that fish reaching its genetic possible age are low.

This is also why triploids are popular in food production; all they do is grow. They don't develop gonads, which is a waste of energy for an organism that is supposed to only be eaten.