There is an assumption that the more other animals are like humans the greater the moral imperative not to exploit them. This exemplifies humanocentric, speciesist reasoning. Just because they are different to us in some ways does not mean they are not sentient. Just because we cannot hear their screams does not mean that they do not suffer at our hands. Even if their deaths were painless, we do not have a right to take their lives from them.
Unfortunately, for them, they do not just lose their lives; they lose them violently. Because fish breathe in water, when they are taken from the water they suffer as humans do when deprived of air. They have pain receptors that enable their conscious awareness of pain, and these receptors are enriched in their mouths, the part of their bodies that is violently assaulted by fishing hooks.
Farmed fish are imprisoned in confined filth where they suffer infection, injury and disease. Like all animals in large farms they are confined in such huge numbers that they have barely enough space to move. Many die before slaughter. The less fortunate are skinned and eviscerated, often while still conscious.