A fish is poikilothermic water-dwelling vertebrate with gills. Fishes are divided into the bony fishes (class Osteichthyes), cartilaginous fishes (class Chondrichthyes), and various groups of jawless fishes, including lamprey and hagfish. Other aquatic animals like jellyfish and cuttlefish are not true fish. Some species of fish have been specially bred to be kept and displayed in an aquarium. Fishes can be found in most all large bodies of water in either salt and fresh water, shallow or at depth.
Fish are an important source of food. Other water-dwelling animals such as mollusks and crustaceans (commonly called shellfish) are often considered as fish when used as food.
Catching fish for the purpose of food or sport is known as fishing.
On May 15, 2003, the journal Nature reported that all large oceanic fish species worldwide had been so systematically overcaught that fewer than 10% of 1950 levels remained. [http://www.globeandmail.com/servlet/story/RTGAM.20030514.wfish1405/BNStory/National/] Particularly imperilled were sharks, Atlantic cod, and Pacific sardines. The authors recommended immediate, drastic cutbacks in fish catches and reservation of ocean habitats worldwide.