Freshwater fish are those that spend some or all of their lives in fresh water, such as rivers and lakes, with a salinity of less than 0.05%. These environments differ from marine conditions in many ways, the most obvious being the difference in levels of salinity. To survive fresh water, the fish need a range of physiological adaptations. 41.24% of all known species of fish are found in fresh water. A fish is defined as an aquatic or marine animal with vertebrae. All fish have vertebra, except sharks and rays that have cartilage. Cartilage is more flexible than bone, but strong enough to support the body. They usually possess gills in the adult stage and have limbs in the form of fins. Fishes also include the jawless vertebrates such as the lamprey and hagfish; and the shark, ray, chimaera, lungfish, and bony fishes. The bony fishes are the most common. A bony fish has jaws that are well developed, formed by true bone rather than cartilage. Fish are very different in appearance, size and shape. This all depends on the environment that it lives in. Fish are part of the ecosystem entering the flux of energy at different levels of the food chain. This book introduces the ecology of fishes by describing the inter-relationships between fishes and the aquatic habitats they occupy. Sequential reading, chapter by chapter, covers the main themes of ecology, including habitat use, species interactions, migration, feeding, population dynamics and reproduction in relation to the major habitats occupied by fishes.