Global genetic stock structure of the copper (Carcharhinus brachyurus) and dusky sharks (Carcharhinus obscurus): Interspecific comparisons and implications for management

Genetic stock structure information is needed to help delineate management units and monitor trade in sharks, many of which are heavily exploited and declining. Among these, the copper (Carcharhinus brachyurus) and dusky sharks (Carcharhinus obscurus) are considered some of the most vulnerable to overexploitation, given their K-selected life history strategy. The International Union for the Conservation of Nature (IUCN) categorizes the copper shark as "Near-Threatened" globally and the dusky shark as "Vulnerable" globally, with the Northwest Atlantic-Gulf of Mexico population qualifying for "Endangered." Global mitochondrial stock structure is assessed for both C. brachyurus and C. obscurus by analyzing part of the mitochondrial control region (mtCR). Samples from 120 copper sharks and 255 dusky sharks were obtained from 8 geographically dispersed locations. For C. brachyurus, I found 20 mtCR haplotypes and detected significant genetic structure between three sampling areas that were separated by oceanic expanses: "Australia/New Zealand (ANZ)," "Africa (AFR)" and "Peru (PER)" (phiST = 0.95, p < 0.000001). For C. obscurus, I found 25 mtCR haplotypes and detected significant genetic structure between three sampling areas: "U.S. Atlantic (USATL)", "South Africa (SAF)" and "Australia (AUS)" (phiST = 0.55, p < 0.000001). Copper sharks showed a major phylogeographic discontinuity between Africa and the two Pacific poulations, which indicates an absence of female-mediated gene flow for millions of years. Dusky shark analysis suggested some recent female-mediated gene flow between SAF and AUS, but not between either of these locations and USATL. Preliminary evidence supports structure between USATL and Southwest Atlantic (Brazil) C. obscurus, suggesting that replenishment of the collapsed USATL population via immigration of females from elsewhere is unlikely. Mixed Stock Analysis (MSA) simulations showed that reconstruction of the relative contributions of sampling areas to shark fins in trade is possible using mtCR sequences. Once global genetic stock structure of these species is fully resolved, region-specific reconstruction of landings from genetic surveys of Asian fin markets will be possible.