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The Northeast Atlantic mackerel is a small fish with grey or greenish-blue scales and tigerlike black stripes from mouth to tail. Lacking a swim bladder the gas-filled organ that helps most fish move up and down in the water the mackerel would sink and die if it ever stopped. So it is always on the move, looking for plankton, crustaceans, and other small fish. It travels in shoals that can be more than 100 feet deep and 600 feet wide, and sometimes, when this great nomadic army catches a glimmer of light, it can resemble an underwater version of the northern lights.