A Review of Historical and Contemporary Evidence for Atlantic Bluefin Tuna (Thunnus thynnus) Spawning Sites

Atlantic bluefin tuna (Thunnus thynnus) are a highly migratory scombrid that traverse the north Atlantic Ocean. During the extensive history of exploitation of Atlantic bluefin tuna as a resource, several paradigms have been proposed in describing population structure. Currently, a two-stock model exists that defines separation between an eastern and western stock, with dominant spawning sites in the Mediterranean Sea and Gulf of Mexico, respectively. The discovery and continued collection of larvae in the Slope Sea renews research interest into spawning at alternative sites. Evidence for spawning outside of the Mediterranean Sea and Gulf of Mexico is well-supported in historical literature. Sites of interest that are included in this review are: (1) historically recognized spawning sites of the Mediterranean Sea and Gulf of Mexico; (2) recently confirmed spawning, in the Slope Sea; (3) plausible spawning sites in the western Atlantic: the Straits of Florida-Caribbean Margin, South Atlantic Bight-Blake Plateau, and Yucatán-Western Caribbean; (4) plausible spawning sites in the eastern Atlantic: the Bay of Biscay, off mainland Portugal, South Macaronesia, and North Macaronesia; and (5) presumed collapsed sites, of the Black Sea, Gulf of Guinea, and Brazil. This review aims to compile references relevant to the likelihood of Atlantic bluefin tuna spawning in alternative spawning sites, including collections of larvae, presence of mature adults in spawning condition, captures of young-of-year juveniles, and supporting evidence in tagging studies, genetics, stock-mixing and population modeling, and microchemistry. Oceanographic conditions of areas of interest, including sea surface temperature, salinity, height anomaly, current velocity, and chlorophyll are described, and regional suitability for spawning and larval survival is discussed. In compiling this review, uncertainties in Atlantic bluefin tuna spawning are highlighted and gaps in knowledge addressed. Future work to resolve questions related to spawning, both spatially and temporally, is crucial for improving our understanding of the population. Sustainable management of this species is reliant upon accurate analysis of their structure and spawning behavior.