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Atlantic mackerel (Scomber scombrus)

Also known as: Boston mackerel, Norwegian mackerel, Scottish mackerel, mackerel

Illustration: Sarah Rot

Morphology & Biology

The most notable feature of Atlantic mackerel (Scomber scombrus) is their iridescent blue-green back with oblique and parallel dark lines. Their bellies are silvery white, so that when predators beneath them look up towards the surface, the mackerel are harder to spot. The zigzag pattern on their backs consists of 23-33 stripes, working as a highly effective camouflage method, mimicking the waves of the ocean. In front of the strongly forked tail fin, there are five small finlets along the top and bottom. The average size of the adult mackerel is 35cm, but lengths up to 60cm are possible. The average weight is 0.5kg, but large mackerels can weigh up to 3kg.

Mackerel lack a swim bladder, making them highly mobile – they can dive to great depths and quickly return to the surface. This manoeuvrability helps them to avoid predators such as the spiny dogfish, porbeagle sharks and tuna. However, because they lack a swim bladder, mackerel have to stay in constant motion to avoid sinking.

The breeding season of mackerel depends on the temperature of the water. In the south of Ireland, mackerel spawn from March-April, whereas in the west of Ireland and north west of Scotland they spawn later, from May-June. Spawning takes place at a temperature of 12-13°C and a depth of 80-120m. The female lays approximately 350-450,000 eggs in the water, which are immediately carried away by the sea current. After five days the larvae hatch and migrate to sheltered coastal areas to develop and grow. After one year, a juvenile mackerel will be approximately 25cm long and weigh around 100g. At the age of approximately three years and a body length of 30cm mackerel become sexually mature. A mackerel can live to be 20 years old.

Schooling mackerels (Biopix / JC Schou)

During the winter mackerel rest and reduce their feeding intensity. Mackerel are able to feed through biting or filter feeding. From spring onwards they work together to feed on plankton using an impressively efficient feeding technique. A densely crowded school of mackerel will swim together with their mouths wide open. As they sweep through the sea in this mouth-to-mouth formation, it is almost impossible for plankton to escape. If small prey such as plankton or copepods try to avoid the mouth of one mackerel, they will almost certainly be captured in the mouth of the next.

After spawning in late spring and summer, mackerel need to regain their strength through foraging for food. In large schools, the mackerel hunt for juvenile herring, sprat, cod and whiting. During this time, mackerel often form huge shoals together with herring.

Distribution

Mackerel populations can be found on both sides of the North Atlantic. Mackerel is a common schooling fish found in the coastal waters of north-eastern North America, around the west coast of Europe, the North Sea, the Baltic Sea (though only sporadically), throughout the Mediterranean, and in the Black Sea.  They are some of the most ecologically and economically important fish species and are often found to be abundant.

Threats

The Atlantic cod has always been very heavily fished. Together with salmon and tuna, cod is one of the most The geographical range in which this fish is captured is increasing. This may be due to climate change, as warming water expands the mackerel’s range northward into Icelandic waters. This means that fishing vessels are able to fish in different nations’ territorial waters simultaneously, using different capture quotas or regulations.

Mackerel are caught with a wide variety of fishing gear, including nets and trawls. Trawlers, purse seines and gill nets all pose significant risk to the environment in which they are used. Many marine wildlife species that are not target species are also inevitably caught, leading to high numbers of bycatch casualties. ‘Ghost nets’ – nets that are lost at sea – are also a serious problem. This is not only due to pollution, but because ghost nets cause ongoing mortality as they drift through the ocean, entangling anything that may come into contact with them.

Fisheries & Aquaculture

Atlantic mackerel are intensively targeted by the fishing industry, primarily through the use of pelagic trawlers and purse seiners. They are a species with high economic importance. Reported catches of Atlantic mackerel in EU waters amount to at least 1 million tonnes each year. It’s important to remember that many fish that are captured are unreported, and many are deemed bycatch. Despite the extremely high capture rate of mackerel, they are considered of least concern by the IUCN, based on an assessment in 2010.

Mackerel schools are extremely important to other marine wildlife. They are an energy-rich food source for predatory fish, marine mammals and seabirds. For this reason they are an integral part of many ecosystems.

Fun fact

Atlantic mackerel are closely related to tuna. Like tuna, they are strong and powerful swimmers. Mackerel can swim 50 metres in just 10 seconds!

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