The Atlantic mackerel is a very common sea fish around the coasts of Britain and Ireland. A relative of the much larger tuna, it is beautifully streamlined and built for speed. Its head is pointed, while its body tapers to a forked tail. The eyes and mouth are large. Near the tail on both the dorsal and ventral surface is a series of four or five small fins. There are two dorsal fins, both of which fold fiat to produce a sleek, streamlined shape and reduce friction while the fish is swimming at high speed.
The mackerel has distinctive colouring -its back is metallic turquoise overlaid by dark, wavy bars. After the fish dies, the colour fades rapidly to dull grey-blue. The flanks are iridescent silver with rainbow tints of pink, gold, purple and blue. The overall colour provides ideal camouflage in the open sea, helping the fish to avoid the attentions of predators.
Swim or sink
Mackerel are active swimmers, and it is said that they never stop swimming. This may help them breathe easier because more water passes through their gills. Another possible reason is that they don’t have swim bladders (an organ which fills with air to allow fish to stay at a particular depth). If they stopped swimming, they would probably sink to the bottom!
Mackerel winter in huge shoals in deep water near southern Ireland and along the Continental Shelf. It’s here that they are most vulnerable to Eastern European and
Soviet trawlers which plunder huge stocks. When spring comes, they move inshore to spawn. They are a schooling fish – that is, similar-aged fish swim and feed together. At night they move near the surface. Mackerel hunt mainly by sight, so if you are fishing for them, look for them in the morning and evening. They swim down to deeper waters in the day-time. Occasionally, schools of small fish may tempt them to feed near the surface during the day.
Like most sea fish, mackerel eat anything that swims or is small enough to be swallowed, including plankton, shrimps, worms, squid, fish eggs and fry. They also browse on the sea bed and eat a variety of molluscs and crustaceans. They don’t grow large, the average length being about 14in (35cm), but fish up to 7 lb (3.2kg) have been recorded off the US coast.
Mackerel are a very popular summer fish. Casting three to five feather baits (on one line) off piers is a popular method of catching them. Other anglers prefer to wade in shallow surf beaches, using light baitcasting rigs with small spinners.
Mackerel spawn from January to June. Each female sheds up to 500,000 eggs which float at the mercy of the current. Every egg contains an oily globule to help it stay buoyant. Recently-hatched larvae feed on their yolk sacs; when the yolk is used up, they sieve plankton through their gill rakers. In 40 days the fry are about 2in (5cm) long.
They are an extremely slow-growing species, perhaps the very slowest. A 1lb (0.45kg) mackerel is about six to eight years old, and a 2 lb (0.9kg) fish is about ten to twelve years old. Larger fish – including sharks – prey on them.