WKCOLIAS aimed to compile and analyse data on the abundance, distribution and biology of Atlantic chub mackerel (Scomber colias) in Atlantic European waters, to explore the connectivity between European and Northwest African waters populations, to propose stock units in the ICES region and to identify stock assessment approaches potentially applicable to those stocks taking into account the data available. The metadata on fisheries, surveys and biological information were compiled across the region from the Bay of Biscay to Senegal. Data from the northern part of the Bay of Biscay and from the Mediterranean Sea will be compiled in the future. The WK concluded that current information on chub mackerel population structure and dynamics is not sufficient to define stocks in the study area. Available data do not point to a continuous distribution from Europe to Northwest Africa (NWA) shelf waters. The species is fished across the area, with most landings by purse seiners from Biscay to central Morocco and by pelagic trawlers in southern Morocco. Landings increased in the last decade in all areas. In European waters, fisheries are unregulated apart from technical measures. A single stock is considered in NWA and is assessed yearly within a FAO-CECAF Working Group. If an assessment of chub mackerel is required in the ICES area, the WK considers that plausible stock scenarios are: (i) a single stock in the Bay of Biscay and Iberia Peninsula waters ecoregion or (ii) two stocks in the above ecoregion separated in the mid-Cantabrian Sea. The time-series of data currently available are sufficient to explore length-based or even age-based assessment models. A good approach would be to combine several methods of different complexity. The main limitations relate to stock identification, knowledge on migration patterns, location of spawning and recruitment areas, and validation of age-determination criteria. The WK recommends a large-scale study of stock identity with a holistic approach, standardization of surveys and biological analyses both within and between European and NW African institutes. Finally, the WK highlights that the increase of landings and uncertainty about stock status, may require policy and management attention.