So the question I would pose is, "How could you reverse/slow that trend by selection in your balanced pond?" Is it possible to do it? What is(are) the practical method(s) that can accomplish that goal?
I think a grow-out or brooder pond would be an excellent resource.
Take the aggressive and larger than their class peers fish that you catch, and put them in your small brooder pond. (Let them spawn in there and then stock a few of those fish back into your main pond.)
Use the traits of the aggressive shooters to improve your population genetics, rather than the opposite - which probably occurs in most fishing ponds.
