Not easy to start a GShiner population in an established LMB pond from only a few bait leftovers. My guess is the prior owner stocked a bunch early and they have survived as a ongoing population. That is a good thing for a LMB/BG pond. I agree with Theo that one has not been there for 5 years. Probably hatched in the pond 2yrs ago. It would be good if you could keep the population going. Has the pond refilled?