There are two separate populations, eastern and western, with little or no interchange. Separate stock assessments are conducted for each (ICES and DFO+NOAA).

Eastern Atlantic
The east Atlantic Mackerel stock is currently considered to comprise three main unit stocks (southern, western and North Sea), with variable proportions of these three mixing in the northerly feeding grounds. The southern component spawns in Spanish and Portuguese waters, and the western component spawns in the Bay of Biscay and northwards around Ireland and west/northwest of the U.K. The eastern Mediterranean populations (Greece, Italy) are separated genetically from the western Mediterranean stock (Barcelona) which forms a panmictic unit with eastern Atlantic populations (Zardoya et al. 2004). The third component spawns in the North Sea and Sakgerrak. Although the North Sea spawners form to some extent a discrete unit from the other ones, commercial catches cannot be allocated to individual stocks, and therefore assessments are undertaken for the combined stock (ICES 2006).

For the Western Component, estimates of the spawning stock biomass (SSB), derived from egg surveys, indicate a decrease of 14% between 1998 and 2001 and a 6% decrease from 2001 to the 2004 survey. The results from 2007 indicate a 5 % increase from 2004 to 2007 (STECF 2009). For the North Sea Component, the 2002 and 2005 triennial egg surveys in the North Sea both indicate similar egg production, but in 2008 it has decreased by about 40% (STECF 2009). For the Southern Component, catches increased from about 20 000 t in the early 1990s to 44 000 t in 1998, and were close to 50 000 t in 2002. Estimates of SSB, derived from egg surveys, are highly variable, and give average estimates of around 16% of the combined North Eastern Atlantic mackerel stock (1995–2007) (STECF 2009). The main catches are taken in the North Sea (ca. 50%), the Norwegian Sea (10%), west of UK and Ireland (15–20%), south of Ireland and in the Channel (ca. 15%) and the rest in the southern area.

Based on the most recent ICES estimate of SSB in 2009, the eastern stock is classified as having full reproductive capacity. Fishing mortality in 2008 is estimated to be just above Fpa (precautionary fishing mortality). SSB has increased by 47% since 2002 and is currently estimated to be above Bpa (precautionary biomass). The 2002 year class is the highest on record. Subsequent year classes are estimated to be about average (STECF 2009). The SSB is expected to remain stable in 2011 for a catch in the range of 527,000–572,000 tons (STECF 2009).

Western Atlantic
The northwest Atlantic Mackerel stock has been evaluated jointly by NOAA in the US and DFO in Canada (TRAC 2010). There is a high degree of uncertainty in this assessment, and available data do not allow for estimation of biological reference points. The current recommendation is that catch not exceed the average total landings of 2006–2008 (80,000 mt) until more information is available (TRAC 2010).

Based on a generation length of between 3.5 and 6.5 years (Western and Eastern Stocks respectively), decline can be measured over a period of 11–20 years. The TRAC 2010 report shows a SSB decline in the Western Stock of approximately 53% over 20 years (1987–2008) and a 33% over 11 years (1997–2008). Based on the ICES 2009 assessment (ICES 2009), the eastern stock SSB has declined 10% over the past 20 years (1987–2008), with no decline over the past 11 years (1997–2008). Declines were calculated based on linear regression of SSB biomass over time. The most recent stock assessment (ICES 2009), the SSB in the eastern Atlantic was estimated to be 1,670,000 tonnes. The SSB for the western Atlantic stock is estimated at 144,000 mt tonnes (roughly ten times smaller) (TRAC 2010). Based on these proportions, there has been a total SSB decline of between 3 and 14% over the past 11–20 years across the global range of this species.

Decreasing