I have had green sunfish and mosquito minnows just show up pretty fast in new ponds that I have dug, with no introduction of any fish on my part.
How did they get in there?
How did the WATER get there?
Is there any change a tiny portion of the water came from a water source containing fish?
Purely for discussion, I had just the opposite experience of yours this fall. I did some work by our creek and was getting eaten by mosquitos. I never get mosquitos down there because the creek is full of hungry Gambusia. However, our creek went dry this summer due to the drought. We did get some fall rains that raised the groundwater level. Eventually some of the low spots in the creek did fill with water percolating upwards from the creek bottom, but looking at the debris in the creek, it was obvious that the creek had never resumed flowing. I was working by one such pool and it had damn mosquito larva swimming in it. I sure look forward to my Gambusia coming back this spring!
