Exploring Atlantic chub mackerel Scomber colias (Gmelin, 1789) population structure across the Atlantic and Mediterranean using phenotypic traits

Efficient fisheries management relies on understanding population structure and dynamics of the target species. This study investigated the population structure of the Atlantic chub mackerel ( Scomber colias ), an economically important pelagic species widely exploited across the northeastern Atlantic and Mediterranean, but with no formally defined stock units. The currently proposed fisheries management units suggest either a single unit or two units for the Bay of Biscay and Iberian waters, with a potential division between European and African waters, but lack formal comparison to assess biological consistency. Samples from eight Atlantic locations, ranging from the Cantabrian Sea to central Morocco, and three Mediterranean locations were compared through otolith shape analysis, body morphometry, and gill raker counts. The results from different methods broadly aligned with current recommendations for regional fisheries management, although each method captured population differences at distinct temporal scales. Body morphometric differences suggested higher spatial heterogeneity in Atlantic waters, distinguishing among Cantabrian, Iberian (west and south), African (central Morocco and Canary Islands), and Atlantic islands (Madeira and Azores) samples. Further, differences were found between Atlantic and Mediterranean waters, as well as between western and eastern Mediterranean samples. Gill raker counts further suggested potential distinct source populations in the Cantabrian Sea, Iberian waters (including the western Mediterranean), west African waters (extending to all Atlantic islands), and the eastern Mediterranean. High inter-individual variability in S. colias otolith shape limited the power to detect spatial differences and highlighted the need for increased sampling effort for future studies.