Fish has to extract oxygen dissolve in the water in which they live. Oxygen dissolves better in cold water than in worm water gaseous exchange on fish occurs through the gills.The gills are found it back of the mouth cavity in the gill chamber which is cover by a moveable operculum. There are fout pairs of gills, each gill is made up of two roles of gill filament attached to a bony gill bar blood vessels run along the gill bar an small capillaries ranch off from blood vessels. Gills rakers sieves out particles from the water before it flows over the delicate gill filament and they form an important part in the feeding mechanism of the fish.

The gill filament have an enormous surface area for gaseous exchange an the gill plate increases this surface area also. The gill filament lies on top of each other like the pages of a book. Onless they are kept a apart by a continuous flow. The surface area is greatly reduce and is inadequate to muet the gaseous exchange needed by the fish.

(DIAGRAM OF THE FISH RESPIRATORY SURFACE)