Geographical expansion of Northeast Atlantic mackerel (Scomber scombrus) in the Nordic Seas from 2007 to 2016 was primarily driven by stock size and constrained by low temperatures

In the mid-2000s, summer feeding distribution of Northeast Atlantic mackerel (Scomber scombrus) in Nordic Seas began expanding into new areas. The present study explores how spawning stock biomass (SSB) and environmental conditions influenced this expansion using data from scientific surveys conducted in Nordic Seas from 1997 to 2016. During that period mackerel distribution range increased three-fold and the centre-of-gravity shifted westward by 1650 km and northward by 400 km. Distribution range peaked in 2014 and was positively correlated to SSB. Mackerel was present in temperatures ranging from 5 °C to 15 °C, but preferred areas with temperatures between 9 °C and 13 °C according to univariate quotient analysis. Generalized additive models showed that both mackerel occurrence and density were positively related to location, ambient temperature, mesozooplankton density and SSB, explaining 47% and 32% of deviance, respectively. Mackerel relative mean weight-at-length was positively related to location, day-of-year, temperature and SSB, but not with mesozooplankton density, explaining 40% of the deviance. We conclude that geographical expansion of mackerel during the summer feeding season in Nordic Seas was driven by increasing mackerel stock size and constrained by availability of preferred temperature and abundance of mesozooplankton. Marine climate with multidecadal variability probably impacted the observed distributional changes but were not evaluated. Our results were limited to the direct effects of temperature, mesozooplankton abundance, and SSB on distribution range during the last two decades and should be viewed as such.