One male and one female BG will find each other and spawn even in 30 acres. Most often the number that survive depends on how many other BG are in the pond. The type of predation will also affect survial of hatchlings. The fewer existing BG there are, the more that tend to survive from each hatching. To a certain extent they are self-limiting in terms of pounds per acre once they become abundant. Dense numbers of BG eat lots of BG eggs and fry.
