Tagging experiments have demonstrated that after spawning, fish from Southern and Western areas migrate to feed in the Norwegian Sea and the North Sea during the second half of the year. In the North Sea the mackerel mix with the North Sea component. Since it has not been possible to allocate catches to separate stock components, all mackerel in the Northeast Atlantic are considered to comprise a single stock (i.e. the mackerel in the Northeast Atlantic stock). Catches cannot be allocated specifically to spawning area components on biological ground. However, to keep track of the development of spawning biomass in different spawning areas, mackerel in the Northeast Atlantic stock are divided into three area components: the Western Spawning Component (Bay of Biscay, around Ireland and west/northwest of UK), the North Sea Spawning Component, and the Southern Spawning Component (Spanish and Portuguese waters). Interannual migration pattern vary and is unpredictable.
Mackerel is a typical zooplankton feeder on the abundant copepod Calanus finmarchicus, but also feed on molluscs (Limacina retroversa) and small fish that can be swallowed, mainly sandeel, herring and sprat.
The spawning stock biomass (SSB) has increased since 2002, and in 2009 the International Council for the Exploration of the Sea (ICES) classified the stock as having full reproductive capacity. However, the mackerel stock is at present harvested at increased risk because the total catches probably are exceeding the TAC and the precautionary levels. The 2002 year class is the strongest in history, while the subsequent year classes have shown closer to normal variability around the long term average.
Scientific analyses in 2008 provided new estimates of several reference points for mackerel, including an increased precautionary approach fishing mortality (Fpa) from 0.17 to 0.23. In October 2008 Norway, EU and the Faroe Islands agreed to implement a new long term management plan, which shall limit fishing to an amount consistent with a fishing mortality rate (F) between 0.20 and 0.22, when SSB is above 2,200,000 tonnes. If SSB is lower than 2,200,000 tonnes, F should be reduced progressively. ICES concluded that the plan is precautionary under the assumption that the TAC equals the total removals from the stock.
Main fishing nations
The main fishing nations are historically EU, Faroe Islands, Russia and Norway. Furthermore, considerable amounts of mackerel have been taken in the Icelandic zone in recent years through a initially unregulated fishery. Norway and to some extent Scotland take most of the catches by purse seiners while the other nations mainly apply pelagic trawl. The Norwegian fishery is predominantly carried out in September-November, and the main parts of the catches are taken in the northern part of the North Sea.
The stock has been measured since 1972. During this period the catches have been assumed to fluctuate between 360 000 and 930 000 tonnes. The main catches are taken in the North Sea, The Norwegian Sea, west of UK and Ireland, south of Ireland, in the Channel and in the southern area.
Regulations
Since 1999, the fishery has been regulated according to agreements between the Faroe Islands, EU and Norway. However, only Norway and EU have been able to agree on a coastal state agreement for 2010 and 2011, and negotiations with Faroe Islands and Iceland are to be continued. National quotas are set for the different fishing fleets, by areas, by minimum landing size and by season.
Stock status and setting of quotas
The spawning stock biomass in relation to precautionary limits is presently in a good condition. There are considerable uncertainties in the catch levels, including unaccounted mortality due to slippage and discards. SSB is estimated to be between 2.63 and 3.34 million tonnes and fishing mortality is estimated to lie between 0.20 and 0.30 in 2009. The SSB estimate is sensitive to the egg survey estimates. This value is preliminary and may be revised in 2011. Catch statistics are an important input parameter in the stock assessment. Earlier analyses demonstrated that the unaccounted mortality is significantly higher than that officially reported. Several measures have been adopted to improve control, and the problem of unreported catches has decreased in recent years.