I'd just like to add that cover/habitat that individual fish species like is an important part of the equation. What is the cover/habitat like in the ponds? Also, try varying your fishing technique for the fish, try casting a small curly tail jig with no bobber, maybe a small crankbait, etc. Like ewest said, fish get used to being hooked and released, so if you don't vary your fishing habits, you could very well see less and less catchable fish, even though they are there.
