We have known for some time that fish grow faster in a dam (Americans read pond/lake) than they do in the hatchery. A hatchery you see, is a business enterprise. And a business enterprise had best watch its feed conversion ratio if it wants to be successful. In other words the level of feeding required for an optimum feed conversion ratio, is not the same thing as the feeding required for maximum growth rate.
Stocking Trout is such a fickle thing.
If one researches the stocking rates recommended on the internet, as I have done, the answers are as varied as the size of the flies in your fly box.
Generally one stocks more if you are putting in little fish (fry to perhaps 3 inches), and fewer if you are stocking larger ones (say above 5 inches). This rate of stocking for different sizes is of course on a continuum from “fry” (being something that has only just absorbed its yolk sac) to fish of 10 inches or so.
By this we take it that larger fish have a higher survival rate. This assumption should possibly be questioned, or at least explored in more detail.