Fish from outside sources can be vectors of parasites and bacteria that can reek havoc on established populations, and there is questionable legality in some states when moving from public waters. They also aren't suited to a pond environment and have trouble getting by in ponds with established predator populations. Fish from hatcheries have traits that have been selected over time for use in ponds. SMB and established bluegill populations do not co-exist without intensive management of the bluegills.