400 is an awful lot of catfish for a 3/4 acre pond. I would immediately start removing some fish for the table. They are at a prime size if they are 1.5 to 2 pounds. Do not return any you have caught as they become extremely hook shy and darn near impossible to catch.

It could certainly have to do with spawning as to why they are not as noticeable. In some catfish ponds, the cats never really learn or desire to completely surface feed. They just grab that pellets that get so water logged and begin to sink.

Feed your fish as normal and then after the feeding, catch a couple cats. Gut them out and check stomach contents, you'll quickly see whether they are taking pellets or not.

Is there a chance you are still having a poaching problem? Do you live at the pond? Any security measures in place(cameras, posted signs, fencing, land mines)?