A well run fish hatchery is a very efficient, well-planed operation. The fish must be ready & grown by the time stockings come around. Disease & parasites must be kept out at all costs. All in all it's a lot of work by highly trained experts.
These days hatchery workers have more knowledge & technology available to them than before, but overall the general process of raising fish is the same as it has been for decades. In the video below, from the 1950's, we look at the Pennsylvania Fish & Boat Commission's different hatcheries & workings as they were back then. I think you'll find this both informative & entertaining.