Fishes are so exquisitely sensitive to stimuli that any pain they experience may be acute. The hook is then pulled from its mouth, worsening the original injury. Many fish are left to asphyxiate slowly. The trauma of suffocation is worsened as the pressure exerted on the body of the fish by air is lower than that exerted by water. This leads to changes in blood pressure in surface veins and internal bleeding in the gills.
Many fish die of exhaustion even before they are hauled in.
People would be horrified if a dog or cat had a hook stuck in its mouth and was dragged across the road to be drowned. However this is exactly what is done to fish without any thought. After the catch Medium sized and large fish are stabbed with a hand-held hook or gaff. Sometimes fish are skinned alive. People think that catching then releasing a fish is harmless but this inflicts terror and pain and often permanently disables or kills the fish.