Genetic Structure of the Gray Reef Shark (Carcharhinus amblyrhynchos), Based on Microsatellite and Mitochondrial DNA Analyses With Implications For Management

The gray reef shark ( Carcharhinus amblyrhynchos ) is an Indo-Pacific, coral reef associated species that presumably plays an important role as apex predator in maintaining the integrity of coral reef ecosystems. Populations of this shark have declined substantially in some regions due to over-fishing, with recent estimates suggesting a 17% decline per year on the Great Barrier Reef (GBR) and projections of only 0.1% of current populations remaining after 20 years at current exploitation rates. There is no information on population structure of gray reef sharks to aid in their management and conservation. We are assessing genetic structure in this species by using entire mitochondrial control region sequences and 15 nuclear microsatellite loci as markers. 275 gray reef shark samples were obtained from across the species’ Indo-Pacific distribution: Western Indian Ocean (Madagascar/Seychelles), Eastern Indian Ocean (Cocos (Keeling) Islands, Western Australia), Central Pacific (Hawaii, Palmyra Atoll, Fanning Atoll), and Southwestern Pacific (Eastern Australia - GBR). Mitochondrial and microsatellite data concordantly identify Hawaii, the western Indian Ocean and Cocos (Keeling) Islands populations as genetically distinct relative to other sampling locations. Interestingly, the Palmyra and Fanning Atoll sharks, although showing significant genetic differentiation from the geographically closer Hawaii population, are not genetically differentiated from the geographically farther GBR population. Overall, at least four genetically identified management units appear to exist despite the modest geographic sampling depth: 1. Western Indian Ocean, 2. Cocos (Keeling) Islands, 3. the Southwestern Pacific/Palmyra-Fanning Atolls, and 4. Hawaii. These results show strong genetic differentiation exists in gray reef shark populations separated by expanses of open ocean, and suggest proper management of this declining species will have to occur at the very least on a regional geographic scale.