Highly migratory Atlantic bluefin tuna (ABFT) is managed as two stocks, Western and Eastern. Western ABFT spawn mainly in the Gulf of Mexico (GOM) and Eastern ABFT in the Mediterranean Sea (MED) (1). Understanding connectivity between ABFT populations is important for conservation and management of this valuable fishery resource that has been exploited for centuries. ABFT are highly mixed, with multiple disciplines supporting weak structuring between Western and Eastern stocks (1). Concerning genetics, subtle structuring of ABFT populations across the Atlantic Ocean has been the conclusion of studies describing genetic tools for traceability (2,3). Larval fish provide the genetic signal of successful breeders and have occasionally been genetically characterized with juveniles (young-of-the-year, YOY) collected in nursery areas. For the first time, cooperative field collection of tuna larvae during 2014 in the main spawning area for each stock enabled us to assess the structuring of ABFT genetic diversity in a precise temporal and spatial frame exclusively through larvae (5). Partitioning of genetic diversity at nuclear microsatellite loci and in the mitochondrial control region resulted in low significant fixation indices. Individual-based clustering analysis of larval ABFT genetic diversity indicate apparent connectivity between the GOM and MED spawning grounds that could support the hypothesis of mixing of breeders belonging to different stocks.This collaborative study was supported by "ECOLATUN" PROJECT CTM2015-68473-R (MINECO/FEDER) funded by Spanish Ministry of Economy and Competitiveness; "TUNAGEN" project funded by IEO; and "BLUEFIN" project financed by IEO and Balearic Island Observing and Forecasting System (SOCIB). This research was funded by NASA (NNX11AP76G S07), the NOAA National Marine Fisheries Science Service through the Southeast Fisheries Science Center, as well as by Cooperative Institute for Marine and Atmospheric Studies under Cooperative Agreement NA15OAR43200064 at the University of Miami. There was no additional external funding received for this study. The scientific results and conclusions, as well as any views or opinions expressed herein, are those of the author(s) and do not necessarily reflect those of NOAA or the Department of Commerce
