Abstract: The impacts of climate change on marine ecosystems can be seen in the changing distribution, migration, and abundance of species in the oceans. For some species this changing environment may be beneficial and can support population expansions. In the North Eastern Atlantic (NEA), the Atlantic mackerel (Scomber scombrus) is undergoing an increase in stock size accompanied by changing summer migration patterns which have resulted in an expansion further north and north-west than previously recorded. This study uses microsatellite loci to confirm the differentiation among North East Atlantic (NEA) and North West Atlantic (NWA) mackerel spawning populations and to assess the level of structuring within these populations. Additionally, in order to enable population specific exploitation rates to be factored into fisheries management, we identified the origin of individuals composing the expanding feeding aggregations in the Central North Atlantic (Greenland, Iceland, Faroes), with all aggregations tested originating from spawning populations in the North East Atlantic. This study showed that microsatellite loci might be useful to assess the contribution of NEA and NWA populations to mixed feeding aggregations across the North Atlantic for large pelagic fish stocks but were not powerful enough to evaluate the specific contribution of known stocks within NEA and NWA. Atlantic mackerel microsatellite allele data for 14 loci and 1231 individuals.