My parents use to own a fish shop. and we sold fresh fish from the market etc as well..
We used to go to the morning fish markets and get our supplies. Red Cod (or the more marketable name Akaroa cod) is still right up there in price.
Commercial fisherman dont discriminate what goes in the bins. Some have huge lice and worms on them. But generally they stick to the skin and gills but never seen any in the actual flesh.
Another one that alot of people love is stargazer/monkfish or sometimes called deep sea cod in the shops. Its slimy ugly and often stink as well. But as long as the public dont see what it looked liked before the fillets came off. They love it.
I speared one out at Titirangi over summer and that had a whole kahawai in it that smelt like it had been rotting in its guts for weeks.