Here in NW tennessee it is common practice for farmers to build "gulley plugs" in their fields as an erosion control measure. Soil conservation is a big deal here. About a year and a half ago, a farmer friend of mine had one go bad on him and instead of building a slow draining basin, he essentially built a pond. He has been trying to drain it several different times. Finally got it done this week only to find thousands of 4-6 inch bluegill in it and hundreds of monstrous crawdads. Pond at its biggest was never more than 1/2 acre, usually smaller, and just a mudhole at that. How in the world do those fish get in there? There is ZERO chance of them washing in from elsewhere. I've heard egrets and other birds play a part. Your thoughts?

How close to a running creek with water was the pond? Is there any other water bodies close to the pond? If water bodies are close by to the pond is there any connecting paths form the water bodies to the pond? If there are no water bodies close to the pond then there is a good chance that eggs of Bluegill were transported to the pond by birds. Also there is a possible chance that at one time someone may have released Bluegill into the pond and they reproduced. Just my two cents.

Plus 1 on what Banker said. In 2011, the majority of the ponds in my area went dry due to the drought but I can catch green sunfish, and often native bluegill, in every one of them.

I was out at the golf course after a heavy rain. Tons of small redear had overflowed the pond and dyed. I expect a large portion made it to the pond down below.

Plus #2.

This little pond definitely has no water above it, whether a creek or pond. Just run off from maybe 50 acres of open pasture and a few acres of woods. The farmer didn't put them in and no one else has access to it. It's not in a location anyone else would have put them in either. I'm always amazed how nature does things... until I saw this I was sure when I had heard about birds (or whatever) "stocking" ponds like this, I had my doubts. Anyway, pretty cool deal.

It is an interesting phenom. No one really knows the answer but lots of theories about birds, turtles, etc.

Most likely it comes from other humans.

Migratory Birds

Got to agree with the humans side. However the bird theory seems likely also.