From www.peta.org , Vegetarian Starter Kit:
"Like other animals, fish feel pain and experience fear. Dr. Donald Broom, animal welfare advisor to the British government, says, "Anatomically, physiologically, and biologically, the pain system in fish is virtually the same as in birds and mammals." When they are dragged from the ocean depths, fish undergo excruciating decompression -- the rapid pressure change often ruptures their swimbladders, pops out their eyes, and pushes their stomachs through their mouths. Then they're tossed onboard ships, where many suffocate or are crushed to death. Others are still alive when their throats and bellies are cut open. Now that commercial fishing has basically emptied the oceans of "target" fish, the seafood industry has turned to raising fish in contained farms, a practice known as "aquaculture," which uses either tanks on land or cages in the ocean. The fish are packed so tightly together that they constantly bump into each other and the walls of the enclosure, causing painful sores and damage to their fins. The enormous amount of feces in the enclosures leads to rampant outbreaks of parasites and disease. In order to keep the fish alive in such unhealthy conditions, large quantities of antibiotics and other chemicals are poured into the water. When the fish are fully grown, they are killed by having their stomachs cut open or die of suffocation when the water in their tank is simply drained away."