Adult Atlantic mackerels form schools close to the surface; they have to swim constantly as they 'breathe' by a method known as ram ventilation, which requires a constant flow of water across the gill surfaces (5). They are active mainly in the day (2), and feed on small fishes such as sand eels (Ammodytes spp.), as well as small crustaceans, which are filtered from the water (4). They spend the winter in deep water, and stop feeding at this time (4); they migrate closer to shore during spring (2). During spawning, eggs and sperm are released into the sea. Both the eggs and larvae are pelagic; the eggs have a globule of oil, which keeps them afloat in the surface waters (4). Larvae begin to feed on copepods (tiny crustaceans) when they reach sizes of around 3mm. They will have grown to lengths of 25 cm after just one year. This is a long-lived species; the maximum recorded lifespan in the North Sea is 25 years (4).