1/2 acre isn't too bad, but it would probably take two or three people to work that big of a net, Someone with catfish or talapia farm experience would be able to remove most in just a few hours, I've seen where some have been fished out of the wild where they have accidently gotten released but for the most part they are very hard to catch that way and can easily be hurt bad.
If they come up to feed in a group it might be possible to net some out but with them getting to that size they are going to spook easy and your not going to get but just a few chances at it at best.