Okay, I think that adding the dock, and any structure, when you lower the water to eliminate unwanted fish makes good sense. I was more concerned that you had a source of GSF continually replenishing your pond via a creek, or drainage from another pond somewhere. I still feel that if the fish you speak of are, in fact, GSF, that not nearly enough time has passed for the HBG to retrograde that far, in those numbers.
By the way, a 1 pound HBG is a nice fish! Congrats!

Did the previous owner live near the pond? Sometimes, well-meaning neighbors and friends will "help" stock a pond with fish from another source, or BOW. An unoccupied pond can be an invitation to everyone who knows it's there, and it just takes one angler emptying his minnow bucket to introduce an unwanted species.

"Forget pounds and ounces, I'm figuring displacement!"

If we accept that: MBG(+)FGSF(=)HBG(F1)
And we surmise that: BG(>)HBG(F1) while GSF(<)HBG(F1)
Would it hold true that: HBG(F1)(+)AM500(x)q.d.(=)1.5lbGRWT?
PB answer: It depends.