Theo…comments from a southern pond owner...
I've had by far the best survival rates for HSB that are released in the deepest area of my pond rather than the shallow areas. As EWEST's material suggests, the critical time for predation is that initial time period that it takes for the HSB to acclimate in your pond. The deep water gives them an extra edge to acclimate before LMB attacks. When released in shallow water areas, regardless of sizes I have dealt with, they are easy prey to LMB. After they get acclimated, it is my belief that survival is very high even among 4-6 inch stockers...at full speed the HSB is one fast pond fish.
Regarding time of year, that may be influenced somewhat by the available forage base you have. For example, if Tilapia are present, then fall is a great time to take advantage of that. Not only for providing a food source for the HSB but also taking pressure off them from predation...the LMB are also focusing on Tilapia.
My best results, having tried spring and fall and different sizes, have been fall stocking in deep water. With the forage base in my ponds, even the smaller HSB have very acceptable survival rates.
