While electronic tagging studies reveal migratory patterns and infer population structure, only molecular genetic analysis can determine conclusively whether populations of Atlantic bluefin tuna are genetically distinct. Early genetic studies comparing genetics of bluefin in the eastern Atlantic to the western Atlantic often failed to show differences among these populations because many of the sites sampled are now realized, through electronic tagging studies, to be mixing zones for both Gulf of Mexico and Mediterranean populations of Atlantic bluefin. Because the east coast of North America serves as a feeding ground for both Gulf of Mexico and Mediterranean bluefin, fish caught off the coast of North Carolina will not show significant genetic differences from Mediterranean fish, because some of them are from the Mediterranean.

Refining the sampling scheme to only include fish that have visited a known spawning ground has revealed a different story. Tag-A-Giant Foundation scientists have examined population subdivision in Atlantic bluefin tuna by sequencing 860 base pairs of the control region of mitochondrial DNA (mtDNA). A total of 178 samples obtained from the known spawning grounds in the Gulf of Mexico and Mediterranean Sea were analyzed. Bluefin tuna populations from the Gulf of Mexico and the Mediterranean Sea were found to be genetically distinct based on Φ_st, sequence nearest neighbor and AMOVA analyses, supporting the hypothesis that these two major spawning areas are independent stocks. However, small sample sizes and the relatively recent (~500,000 years ago) divergence of the populations from a common ancestor have detracted from the significance and reproducibility of the mtDNA results. Analyses of microsatellites have allowed us to overcome some of the limitations of mtDNA and obtain even more intriguing results. 

Microsatellites are regions of DNA that consist of rapidly mutating, repeated sequences of nucleotide bases, the building blocks of DNA. Examining DNA microsatellites is helping to distinguish bluefin populations and track their distribution in the sea. Microsatellites are inherited from both parents and thus reflect the movement of both males and females among populations. In contrast, mtDNA is only inherited from the mother. Recent studies at the Virginia Institute of Marine Sciences have used microsatellite loci in larvae and young of the year (for which birth-place is known with confidence) to confirm mtDNA findings that bluefin form unique populations in the Gulf of Mexico and the Mediterranean. Interestingly, the faster rate of mutation of microsatellites has also resulted in significant genetic differences within the Mediterranean basin as well. The data suggest that Atlantic bluefin tuna are more highly structured at the population level than previously thought, perhaps including a distinct resident population in the eastern Mediterranean Sea.

Introducing genetics to the tagging program has provided a powerful tool for identifying population discreteness and mixing and for determining spawning grounds. However, it is still impossible to accurately determine the population of origin of an individual bluefin tuna caught in the North Atlantic. We hope to make this possible in the near future by developing a population identification test using microsatellites.

Currently, the only tagged fish that are assigned to a population 1) were tagged in a known spawning area; or 2) have a visit to a known spawning ground recorded in their track. DNA from these individuals are currently being analyzed to create reference populations for the Gulf of Mexico and Mediterranean Sea. The goal is to identify consistent similarities and differences among fish from the same spawning ground and between fish from separate spawning areas, respectively, to develop a diagnostic test for Gulf vs. Med origin. The next step will then be to examine the DNA of fish for which we have no origin data, including tagged fish still at-large and tag returns from juveniles who had not yet entered a spawning ground, to assign each fish a population of origin. Hopefully in the near future any fish caught by a fisherman or found in a market can be assigned to a spawning stock.

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