Changes in Growth of Northeast Atlantic Mackerel in Relation to Spawning Stock Biomass and Environmental Condition

Northeast Atlantic mackerel (Scomber scombrus) stock is one of the most important commercial fish stocks in European waters. Majority of mature mackerel feed in surface layer of Nordic Seas during summer and aggregate in the Northern North Sea in fall. Annual average weight-at-age for 3 to 8 year old mackerel, collected in September and October, from 1984 to 2013, ranged from 270g to 720g. All age classes followed a similar trend of declining weight from 2002/4 to 2013. Weights were at a record low from 2010 to 2013. In 2013, mackerel weight-at-age was 26% lower than the overall mean for the study period. Similarly, mackerel condition declined continually from 2005 to 2013 and decline was more pronounced for larger fish (36-39cm) than smaller ones (32-35cm), 12% and 6%, respectively. Mackerel condition in fall is an annual growth signal. Preliminary results indicate condition declined when mackerel spawning stock biomass (SSB) increased (F_1,28=23.5, p<0.001; condition anomaly = 45.8 – 9.3*10^-6*SSB, r²=0.46) and when Atlantic Multidecadal Oscillation (AMO) index increased (F_1,28=9.0, p=0.006; condition anomaly = 9.4 – 168.0*AMO, r²=0.24). This suggests mackerel condition is density dependent and is mediated via food availability as sea surface temperatures were high (positive AMO index).