Scientific Journal Article Summary

Mitochondrial DNA and Electronic Tracking Reveal Population Structure of Atlantic Bluefin Tuna (Thunnus thynnus)
Andre M. Boustany, Carol A. Reeb, Barbara A. Block
Marine Biology (2008) 156:13-24

Despite decades of research, there are still many unknowns regarding the population structure of Atlantic bluefin tuna.  Although there are no clear barriers to interbreeding and gene flow, fisheries managers treat the species as having two distinct populations – a western that spawns in the Gulf of Mexico and an eastern that spawns in the Mediterranean Sea – primarily based on differences in life history traits such as maturation age. 

In this study, genetic analysis (i.e., Φ_st and sequence nearest neighbor analyses of DNA sequencing of the mitochondrial control region) confirmed uniqueness of not only the Gulf of Mexico and Mediterranean Sea fish but also identified two distinct populations within the Mediterranean – one spawning in the western basin and the other spawning in the eastern.  In fact, the Gulf of Mexico population was more genetically similar to fish in the eastern Mediterranean Sea than either population was to the western Mediterranean , disproving assumptions based on geography alone. 

Genetic diversity was high, with 141 different genetic patterns, or haplotypes, present in the 178 fish analyzed.  Furthermore, Atlantic bluefin tuna were found to be a relatively young species, having originated approximately 135,000 years ago (for comparison, modern humans are thought to have originated about 200,000 years ago).  Because the species is so young, the high level of genetic divergence found in this study suggests that there is currently little to no gene flow among the populations.

This study has several implications for fisheries management.  While validating the separate management of the western and eastern Atlantic stocks, the results suggest the need to also explore unique management regimes for the eastern and western Mediterranean Sea .  Furthermore, classical population theory tells us that the risk of population extinction increases with population isolation.  In the case of Atlantic bluefin tuna, this indicates that the extinction risk of a population, such as the one that spawns in the Gulf of Mexico, is increased; since the population appears to be completely reproductively isolated, the chances of augmenting the population with migrants (e.g., from the western Mediterranean Sea) are slim. 

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