Touch is the primary and most immediate of senses and it is as important to fish as it is to any other group of animals. In fish the sense of touch is most important in those species that live in close association with objects, the sea floor, coral reefs, aquatic plants and in some cases other fish. For many fish the sense of touch is also important during mating.
As in most animals the skin of a fish is an important sensory organ and it is endowed with a mass of sensory nerves. Many of these relate to the fish's sense of touch, but others sense water condition such as temperature and salinity.
Scientific research has shown that fish are highly sensitive to variations in water temperature and that they can detect changes in ambient water temperature as small as 0.003 to 0.007 degrees. This is obviously useful to a fish that is trying to move to a warmer or cooler area.
Marine fish, as might be expected are also highly competent at sensing the salinity (salt content) of the water around them. In scientific experiments fish have been shown to be able to discriminate between salinity levels varying by as little as 0.5 parts per thousand.
Fish are also known to be able to detect small changes in pressure. In fish with swim-bladders the swim-bladder plays an important role in this ability, but fish without swim-bladders have also been shown to be sensitive to small changes in water pressure.