Distribution and biology
Mackerel in European waters are managed as one stock, Northeast Atlantic mackerel, which exist of three spawning components: North Sea mackerel which spawn in the central part of the North Sea and Skagerrak (May-July); western mackerel which spawn west of Ireland and the British Isles (May-July); and southern mackerel which spawn in Spanish and Portuguese waters (February-May).
The mackerel spawn in the surface layers of the ocean. Larvae measures 3.5 mm at hatching and grow to about 20 cm already the same autumn. Mackerel lack swim bladders and therefore have to swim constantly in order to avoid sinking. They are typical plankton feeders and swim with an open mouth to filter plankton through their gills. Mackerel also prey on fish larvae and small fish. The mackerel prefer warmer waters and generally higher temperatures than 6°C. They become sexually mature at about 30 cm in length. The mature part of the North Sea component, which the mackerel along the Norwegian coast belong, overwinters outside the western part of Norway and in the outer part of the Norwegian Trench, reaching north up to Viking bank.
After spawning the western and southern mackerel migrate to the Norwegian Sea, and after a while also migrate to the North Sea and Skagerrak where the mackerel mix with the North Sea mackerel. The southern and western components remain here the whole autumn and further during winter (December-March), until they migrate back to their respective spawning areas.
Mackerel, a pelagic and fast swimming fish, is easy to recognize by its round, completely round shaped and streamlined body. The coloration of green and blue on its back is very characteristic, and from there towards the sides there is a number of irregular cross bands. The body scales are small, and the body itself is soft as silk to touch. The mackerel can reach an age of more than 25 years, close to 70 cm in length and weigh up to 3.5 kg. However, it is seldom we observe individuals larger than 50 cm in length and 1 kg in weight. They possess other typical plankton feeder characteristics as well.
The spawning stock is calculated and measured from their annual egg production. The production of eggs is measured during international scientific surveys throughout the spawning season from February to July. At the same time the numbers of eggs produced from individual females are measured. Research has shown that the same number of females and males spawn. The spawning biomass can then be estimated based on this data. The investigations are highly international and very comprehensive, and these surveys are only performed every third year.
In the North Sea the population is measured using the same methodology, and the surveys are usually performed by Norway and the Netherlands.