Classification of fishes has undergone much change over the last few decades, and further changes are expected, partly because so many groups are poorly known. There are many conflicting hypotheses of relationships, some based on conflicting evidence between morphological studies and molecular studies; however, progress is being made and there are many areas of basic agreement. Fisheries can be categorised into two types-fin fisheries and non-fin fisheries. The former is fisheries of true fishes, whereas the latter is the fisheries of organisms other than true fish like prawn, crab, lobster, mussel, oyster, sea cucumbers, frog, sea weeds, etc. Fin fisheries can be further categorised into two types-capture fisheries and culture fisheries. Accepting that there is a great deal of variety within the world of fish we may wish to ask ourselves what factors control the shape that a fish takes. Many factors are involved but perhaps the most important is where the fish lives, i.e., in water. Water is a very different substance than the air that we live in, it is much more supportive for one thing. This means that it is easier for fish to fly than it is for us, and they can even float, of course we call flying in water swimming, but the fact remains that because they live in water many fish can live their whole lives without ever touching the bottom, or resting on anything. The other side of the coin of this supportiveness is that water is much harder to move through, we can run through air much faster than we can run through water.