Lots of good questions here....experts will weigh in, but a couple questions for you-
What are pond contours-steep sides,depths etc.
Is there any cover to protect yoy HBG/RES?
If you have a vigorous population of HSB, in several size classes, and a pond in which there is little cover for yoy, it's very possible they'll control the offspring, IMHO with one 1/2 acre pond.
