Room 3, of Balclutha Primary School, asks :-

How do fish survive under water?

Malcolm Francis, a fisheries scientist at the National Institute for Water and the Atmosphere, responded.

Like all animals, fish require oxygen to survive. They get it from the surrounding water, but because the amount of oxygen dissolved in water is low, fish need special organs for extracting it: gills.

Gills are feathery structures that are enclosed in cavities behind the mouth, one on each side of the head. Water flows in through the mouth, passes over and between the gills, and then exits through the gill covers. Each gill consists of a large number of fine filaments that contain blood vessels. As blood flows through the gill filaments, it absorbs oxygen from the water flowing over the gills, and then carries it around the whole body.

Fish that require large amounts of oxygen (such as fast-swimming tunas and sharks) have large gills, and need to keep swimming to maintain a high flow of water through the gills. Fish that live in stagnant water, where the amount of oxygen dissolved is very low, also have large gills, and many of these species can actively pump water through the gills (so they don't need to keep swimming).

The oxygen that is extracted from the water by fishes and other aquatic animals is replaced in two ways: it may be absorbed directly from the air (this process is fastest when the water surface is turbulent, for example, when there are large waves, waterfalls or rapids), or it may be produced by seaweeds and the billions of microscopic plants (phytoplankton) that live in the surface layers of lakes or the sea.