RES reach sexually maturity around the 5" mark and a late fall spawn would not be be unheard of. Based on your stocking size I am thinking someone may have bucket stocked your pond or some fish made their way in during a high water event.
I cannot stress enough, this pond has not connected with any other body of water! Also, no one has bucket stocked it either. This pond is in a remote area and I have multiple trail cameras around it.
Thanks for the reply and picture.
