Insights in the Stock Mixing Dynamics of Atlantic Bluefin Tuna in the North Atlantic

Effective fisheriesmanagement requires accurate stock identification, which can be challenging in mixed stock fisheries such as the Atlantic bluefin tuna (Thunnus thynnus). This species is currently managed considering two stocks known to spawn in theMediterranean Sea andGulf ofMexico, respectively. However, recent studies have shown that individuals fromboth spawning components can interbreed at a recently discovered spawning ground, located in the Slope Sea. A better understanding of themixing patterns, as well as the proportion inwhich both stocks interbreed in the Slope Sea are valuable for a reliable Atlantic bluefin tuna stock assessment. With this aim, we assigned genetic origin of 2000 individuals captured at feeding aggregates across the North Atlantic using a 96 SNP panel and analyzed the genetic profile of 500 individuals including 200 potential Slope Sea spawners (i.e., spawning capable individuals captured in this area at the spawning season), using a 8000 SNP array. We confirmed that stockmixing occurs across different feeding aggregates in the North Atlantic, being stronger in the Northwest Atlantic, where the Mediterranean componentwas amajority at some locationswithin and near the Slope Sea spawning ground. The analysis of Slope Sea spawner candidate individuals showed nearly equal representation from both Mediterranean and Gulf ofMexico genetic origin individuals, suggesting similar contribution to the Slope Sea origin offspring. Our findings constitute an important progress towards the understanding of the Atlantic bluefin tuna stockmixing dynamics and the relevance of the recently discovered Slope Sea spawning ground for the conservation of the species.