Eye lens isotopic signatures as indicators of early-life habitats in Atlantic Chub Mackerel Scomber colias (Gmelin, 1789) across the Northeast Atlantic and Mediterranean

Understanding population connectivity in the Atlantic chub mackerel (Scomber colias) is essential for sustainable management, yet its population structure across the Northeast Atlantic and Mediterranean remains poorly resolved. This study presents the first application of stable isotope analysis of the eye lens centre (δ¹³C and δ¹⁵N) in S. colias as a proxy for early-life habitats, complementing genetic, and phenotypic methods. Eye lenses were sampled from multiple geographic regions and cohorts spanning 2019–2023 to assess spatial and temporal isotopic variation. The isotopic composition of the eye lens centre revealed distinct regional patterns consistent with regional environmental and oceanographic heterogeneity. Recurrent patterns, largely congruent across cohorts, included: (i) a distinct isotopic unit encompassing the Bay of Biscay and the central-eastern Cantabrian Sea, (ii) isotopic differentiation of the eastern Mediterranean from Atlantic and western Mediterranean groups, consistent with its oceanographic isolation, although based on limited sampling, and (iii) broad isotopic overlap across Iberian, Northwest African, and western Mediterranean waters, most likely reflecting extensive early mixing. Within the western Mediterranean, the Alboran Sea clustered with Atlantic groups, while the northwestern Mediterranean showed a partial shift in cohorts 2019 and 2020, consistent with variable multi-source recruitment. A Random Forest classifier confirmed the discriminatory power of the approach, with Biscay–North Iberia and East Mediterranean showing consistently high classification accuracy across cohorts. These patterns broadly align with ICES’ proposed stock structure, supporting a Bay of Biscay vs Iberian separation at the mid-Cantabrian Sea, but also suggest permeable boundaries between Iberian and Northwest African components. The persistence of regional isotopic signals across cohorts may suggest that at least a proportion of individuals remain associated with their early-life habitats throughout life, consistent with partial residency in isotopically distinct regions. These findings support the development of spatially explicit, integrative management strategies for this increasingly important coastal pelagic species.