Reproductive ecology as a tool for identifying stock components: the case of the blue whiting (Micromesistius poutassou) in the Northeast Atlantic

Accurate fish stock evaluation requires proper identification of population structure, for which reproductive ecology studies are essential. This study examines blue whiting (Micromesistius poutassou), a widely distributed mesopelagic gadoid managed as a single stock in the Northeast Atlantic, despite ongoing debate about the existence of distinct spawning components. To investigate potential differentiation, we compared reproductive parameters between two regions: the Porcupine area in southwest Ireland (ICES division 27.7.c,k,h,j) and the Peninsula area off the northern Iberian Peninsula (ICES division 27.8.c-9.a.N). Using biological data from 2018 surveys of the Spanish Institute of Oceanography and commercial vessels, we analysed somatic attributes (length, weight and condition) and assessed the species’ reproductive strategy. We also examined interannual variation in Peninsular females’ somatic features, total and batch fecundity, and geographic variation in fecundity and spawning frequency in February–March. Blue whiting was confirmed as a capital breeder, spawning from February to April. Iberian individuals exhibited smaller sizes and lower condition but higher fecundity (mean potential and batch fecundity: 46.817 vs. 12.223 oocytes, 16.808 vs. 6.112 hydrated oocytes), releasing three batches per season at 11-day intervals, compared with one every ~15 days at Porcupine. These findings suggest a distinct southern component, reinforcing the relevance of reproductive studies for stock assessment and management.