Fish are private property we are justified in doing whatever we want to them. Just as you can cut a paper bag into piece, burn it, throw it in the trash, and neglect it in the closet for years.
Fish have no rights. Just like an animate object. What right do you have to tell me I can't do whatever I want to my private property.
Fish farming is a promising way to feed humanity's desire for protein and meat without destroying natural fish stocks or engaging in overfarming generally. Subjecting fish to physical pain is unfortunate, however naturally-caught fish are also subjected to pain, as are many land animals who are grown to be eaten. Fish farming can be seen as a possible evil, however it is likely a lesser evil, and if reasonable actions are taken to minimize suffering, it is justifiable.
No, it is not okay to exploit fish and subject them to physical pain. Fish are animals capable of suffering, so it is against their rights as living creatures to cause them undo stress. New techniques to harvest fish that do not cause these animals undue pain should be established.