I doubt very much that the sunfishes spawned in September in mid-Michigan, although it is not impossible. The small fish more likely came in with pumping water. I doubt that all the bgill died. Since your dad stocked the pond, ask him if he is willing to buy the rotenone to kill it off?

your other "one more thing" questions
- "Do fish always float when they die?" NO.

- "Could they be laying on the bottom of the pond? Yes, and maybe they are decomposed or eaten by crayfish by now.

aka Pond Doctor & Dr. Perca Read Pond Boss Magazine -
America's Journal of Pond Management