Defining Management Units of a Migratory Species: the Global Genetic Population Structure of the Tiger Shark (Galeocerdo cuvier)

The tiger shark ( Galeocerdo cuvier ) is a migratory and globally distributed species, inhabiting warm-temperate and tropical waters. This species likely plays a key role in marine ecosystems and recent evidence of its over-exploitation and population decline in some regions underscores the need for accurate delineation of its population structure worldwide to inform management efforts. We analyzed the global population structure of tiger sharks (n = 289) using 11 nuclear microsatellite loci and sequences of the entire mitochondrial control region (n = 201). Population-level microsatellite analyses revealed strong genetic differentiation among tiger sharks from Atlantic and Indo-Pacific waters (FST > 0.102), and between samples from South Africa and the Southwestern Atlantic (FST = 0.185). There was relatively weak differentiation among sample sites within basins (FST < 0.026). Although individual-level analyses using the software STRUCTURE and BAPS found significant within-basin differentiation, these groupings did not correspond to geographic capture locations, suggesting extensive mixing of adult populations within basins. Preliminary mitochondrial sequence analysis revealed high congruence with nuclear markers, showing strong division of the Atlantic and Indo-Pacific groups. Collectively, these findings imply a strong barrier to dispersal across the South Atlantic, and between ocean basins. In contrast, open ocean expanses appear not to inhibit dispersal across the Indo-Pacific, suggesting an absence of barriers to gene flow across this basin. The detection of mainly basin-wide management units emphasizes that managing and conserving large, migratory species will require collaborative, multi-national and global-scale approaches.