In my neighborhood they are usually fed protein pellets. They net and move to market with no purging. Water quality varies from farm to farm.
Feeding varies as well, for local sales, algae alone is available, but is the lowest priced and would not be acceptable for even the cheapest local restaurant. Pellets fed every other day is a common practice that maintains growth and a pleasant taste.
Vegetable protein pellets were tried by a manufacturer but have been rejected by the farmers for the most part.
I had a poacher hit me real hard, several years ago and stopping feeding the fish stopped him, he was unable to sell them due to the taste.
As for bait I think they will eat anything natural or artificial. If pellet feeding, Stubby Steve is perfect. If not feeding, then try bread a week of so before you intend to catch them, once they start eating the bread, they should be easy to catch.
