Most fish have an organ called a swim bladder, which is filled with gas to make them buoyant. The amount of gas in the swim bladder can be altered so that when the fish swims in deep water, and the pressure squeezes the swim bladder, more gas is pumped into it to compensate. When fish are caught in very deep water and pulled quickly to the surface, their swim bladder expands rapidly and may burst. Some fish, such as sharks and tuna, do not have swim bladders. They have to keep swimming so that they do not sink.
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