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The commercially important Atlantic bluefin tuna (Thunnus thynnus), a large migratory fish, has experienced notable recovery aided by accurate resource assessment and effective fisheries management efforts. Traditionally, this species has been perceived as consisting of eastern and western populations, spawning respectively in the Mediterranean Sea and the Gulf of Mexico, with mixing occurring throughout the Atlantic. However, recent studies have challenged this assumption by revealing weak genetic differentiation and identifying a previously unknown spawning ground in the Slope Sea used by Atlantic bluefin tuna of uncertain origin. To further understand the current and past population structure and connectivity of Atlantic bluefin tuna, we have assembled a unique dataset including thousands of genome‐wide single‐nucleotide polymorphisms (SNPs) from 500 larvae, young of the year and spawning adult samples covering the three spawning grounds and including individuals of other Thunnus species. Our analyses support two weakly differentiated but demographically connected ancestral populations that interbreed in the Slope Sea. Moreover, we also identified signatures of introgression from albacore (Thunnus alalunga) into the Atlantic bluefin tuna genome, exhibiting varied frequencies across spawning areas, indicating strong gene flow from the Mediterranean Sea towards the Slope Sea. We hypothesize that the observed genetic differentiation may be attributed to increased gene flow caused by a recent intensification of westward migration by the eastern population, which could have implications for the genetic diversity and conservation of western populations. Future conservation efforts should consider these findings to address potential genetic homogenization in the species. [ABSTRACT FROM AUTHOR]

The stock structure and productivity components (growth, maturity and mortality) of the Black Sea whiting Merlangius merlangus were studied in the western Black Sea for a year. The results aim to improve fisheries management of whiting in the Black Sea. Fresh samples were collected from commercial bottom trawlers between October 2017 and September 2018. Using monthly length-frequency data set for the growth trajectories, a bootstrapped procedure was employed to improve parameter optimization. Duration and timing of the spawning season and size at sexual maturity were modelled using generalized additive models (GAMs). The presence of clear differences in oocyte size groups indicates that oocyte development of Black Sea whiting is a 'group synchronous' ovarian developmental process characterized by continuous reproduction. From the gonadosomatic index values, their spawning activity intensifies especially between October and April, captured also in the time component in the GAM modelling. The length at first maturity was 13.9 cm for females and 12.5 cm for males. The mortality rates, yield per recruit analyses, and length-based indicators suggest that whiting is heavily exploited with alarming signs of overexploitation. Undoubtedly, setting catch limits and re-establishing management plans will be the first two steps to ensure reproductive success and prevent overfishing (F > Fmsy). [ABSTRACT FROM AUTHOR]