 Key fishery facts Mackerel (Scomber scombrus) is the most valuable pelagic fish species for Scottish Fishermen. The main western waters stock contributes most mackerel to the 64,590 tonnes of Scottish landings, worth £50.4 million. The much smaller North Sea stock remains far below recommended levels and the advice is to protect it. Pelagic trawling is the main fishing method - small quantities are taken by hand-lining. Biology The mackerel (Scomber scombrus) caught by the Scottish pelagic fleet belong to two different stocks - the North Sea and the Western Stock. North Sea mackerel over-winter in the deep water, to the east and north of Shetland and on the edge of the Norwegian Deep. During spring they migrate south to spawn in the central part of the North Sea. The western mackerel stock is found near to the continental slope, over a vast area. These fish spawn between March and July, mainly to the south and west of the UK and Ireland. When spawning is finished, most of the spent fish move to the feeding grounds in the Norwegian Sea and the northern North Sea where they mix with the North Sea stock. Some western stock mackerel, predominantly small individuals, also enter the North Sea through the English Channel. The western stock mackerel travel long distances between the feeding grounds and the spawning areas. In the 1970s and 1980s this movement occurred in late summer and autumn with the fish passing through the Minch. Now the migration occurs later in the year and is further offshore. The pattern of the return northerly journey, after spawning, has remained relatively constant. The boundaries of the spawning areas have also slowly changed, with an increase in spawning activity to the north and west of the area. By the time they reach three years old, most mackerel are mature and at two about half can spawn. Female mackerel shed their eggs in about twenty separate batches over the course of a spawning season. An average-sized fish produces around 250,000 eggs. Juvenile mackerel grow quickly and can reach 22cm after one year and 30cm after two years. The diet of mackerel can vary with the area and the season and consists of crustacean (shrimps) and juvenile fish such as sandeel, herring and Norway pout.   State of stocks The latest ICES assessment in 2007 was a benchmark assessment, incorporating a new triennial egg survey which gave similar estimates of biomass to the survey in 2004. ICES classifies the stock as being harvested unsustainably since fishing mortality in 2006 is estimated to be at Flim =0.26 whereas Fpa is 0.17. Because of the unknown levels of underreporting in the catch, spawning stock biomass in recent years, relative to Bpa, cannot be accurately estimated. But indications are that spawning stock biomass has increased since 2003 and has been stable for the last 3 years at around Bpa.
The stock has been showing much more variable recruitment over the recent years compared to the past. The 2000 and 2003 year classes are estimated to be poor, while both the 2001 and the 2002 year classes are above average. The 2002 year class is estimated to be the highest in the time-series. There is insufficient information to confirm the sizes of the 2004, 2005, and 2006 year classes.
ICES advise that any total allowable catch should cover all areas where Northeast Atlantic mackerel are fished. The EU, Norway and the Faroe Islands agreed a management plan that restricts their mackerel fishing on the basis of a total allowable catch consistent with a fishing mortality in the range of 0.15-0.20. This implies reduced catches in 2008 between 349,000 tonnes and 456,000 tonnes which can in part be attributed to changes in fishing towards larger older fish.
Spawning stock biomass is expected to increase by 4-9% compared to 2007. ICES suggest there are strong indications of substantial underreporting of catches for this stock (60% over a period of at least 14 years up to 2001). In some countries measures have been taken to improve the situation but major uncertainties in landings affect the reliability of the assessment.
Scientific developments in Scotland FRS Marine laboratory plays a major role in the planning, coordination and analysis of the triennial international egg survey. During the 2007 work, additional survey trips were conducted by FRS. Laboratory staff have also been heavily involved in the development of a revised management plan for mackerel. This work is ongoing and will inform how this important species is managed in coming years.
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