Even a study funded by the British Field Sports Society and the National Federation of Anglers found that capture of fish by anglers causes acute physiological stress. They breathe through their gills - taking in water through their mouths, retaining the oxygen and passing it into their bloodstream. Their bodies are covered with overlapping waterproof scales to provide protection against infection without hindering movement. Their scales are not watertight, so fish have a layer of protective mucus, which also keeps out infection. Handling by humans damages this mucus, leaving the fish vulnerable to disease when returned to the water.
Most fish have a row of tiny pores on their sides called the lateral line.
These act as a hearing aid, making them very sensitive to vibrations and reflections. Outside the water, however, they can hear no sounds. They also have a swim bladder to help them balance and move from deep to shallow water. Fish are short sighted. Their eyes can look in separate directions at the same time.