What is a fish and Where do fish live??

Fish are vertebrates that live in water. They first appeared on the earth about 500 million years ago and were probably the first animals to have a backbone. Scientists have given names to nearly 22,000 different kinds, or species, of fish, in many different colors, shapes, and sizes.
Most fish have a spine, or backbone, made of hard bone which snaps if it's bent. These fish are called bony fish. Sharks and rays have a spine made of a tough, rubbery substance, called cartilage, which stretches like elastic. These fish are called cartilaginous fish.
Most fish breathe through a part of their body called gills. Gills enable fish to take in oxygen from the water. Most fish have skin covered with scales. These are thin, bony plates that form a protective covering. Nearly all fish have fins to help them swim. A fish uses its muscles to move its fins.
The smallest fish in the world is the pygmy goby. It lives in the Philippines and, when fully grown, is only 1/2 inch (13 millimeters) long. The world's largest fish is the whale shark, which may grow up to 60 feet

Where do fish live?
More than half of all known species of fish live only in the sea. These are called saltwater, or marine, fish. The porcupine fish is a marine fish. Most other species of fish live only in fresh water—in lakes, rivers, and streams. The sturgeon is the largest kind of freshwater fish.
The salmon is an unusual fish because it spends part of its life in fresh water and part in salt water. Young salmon hatch in freshwater streams before swimming to the sea. Female salmon swim back to the stream where they were born to lay their eggs.
Did you know that some fish can survive out of water? The walking catfish can walk over land to reach water if its own stream or lake has dried up. It pushes itself along the ground with its tail and front fins. Most species of lungfish can live for months in dried-up riverbeds.