[<250 words]
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 emerged
challenging 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 five hundred
larvae, young of the year and spawning adult samples covering the three
spawning grounds and including individuals of other Thunnusspecies. 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 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.