Global phylogeography of the dusky shark Carcharhinus obscurus: implications for fisheries management and monitoring the shark fin trade

Abstract

Genetic stock structure information is needed to delineate management units and monitor trade in sharks, many of which are heavily exploited and declining. The dusky shark Carcharhinus obscurus is a large apex predator that is sought after for its fins and is considered highly susceptible to overexploitation. The International Union for the Conservation of Nature (IUCN) classifies this species as 'Vulnerable' globally and 'Endangered' in the northwest Atlantic. We make the first assessment of global stock structure of C. obscurus by analyzing part of the mitochondrial control region (mtCR) in 255 individuals sampled from 8 geographically dispersed locations. We found 25 mtCR haplotypes and rejected a null hypothesis of panmixia (analysis of molecular variance, Φ ST = 0.55, p < 0.000001), detecting significant differentiation between 3 management units: US Atlantic (USATL), South Africa (SAF), and Australia (AUS). We also found preliminary evidence of population structure between the USATL and southwest Atlantic (Brazil). There were no shared haplotypes between the western Atlantic and Indo-Pacific. These analyses suggest that replenishment of the collapsed USATL management unit via immigration of females from elsewhere is unlikely. Mixed stock analysis (MSA) simulations show that reconstruction of the relative contributions of USATL, SAF, and AUS management units to the Asian fin trade is possible using these mtCR sequences. We suggest avenues for obtaining samples to conduct MSA of the shark fin trade, which could enhance management of dusky sharks and other species that are exploited for their fins.

Genetic structure and diversity of two highly vulnerable carcharhinids in Australian waters

“…C. obscurus attains a large size (Last & Stevens 2009) and is suspected of undergoing long-range temperature-driven migrations on a seasonal basis, with tagging studies revealing an ability to travel considerable distances (Hussey et al 2009, Rogers et al 2013. Our findings of genetic subdivision between eastern and western Australia, however, challenge those of Ovenden et al (2009) and also Benavides et al (2011b), who failed to detect genetic differentiation between these same 2 locations using control region sequence data. We also provide evidence for and against the findings of Ovenden et al (2009) relating to genetic subdivision between Australia and Indonesia.…”

“…Portnoy et al (2010) observed genetic subdivision between eastern and western Australia in C. plumbeus based on mitochondrial DNA (mtDNA), while Ovenden et al (2009) and Benavides et al (2011b) reported evidence for genetic homogeneity between the same 2 regions in C. obscurus. Ovenden et al (2009) also raised the possibility of limited dispersal across the Timor Trench in the latter species through a finding of genetic differentiation between western Australia and central Indonesia.…”

“…Carcharhiniformes are the most represented of the elasmobranchs in the population genetic literature, but few have been examined in any detail (Dudgeon et al 2012). These studies have typically focused on elucidating genetic structure over broad spatial scales, consistently demonstrating large oceanic expanses to be robust barriers to mitochondrial gene flow (Duncan et al 2006, Keeney & Heist 2006, Benavides et al 2011a, also in C. obscurus (Benavides et al 2011b) and C. plumbeus (Portnoy et al 2010). Genetic subdivision on finer scales has also been reported for some shark species, raising important implications for regional fisheries management (Keeney et al 2003, Tillett et al 2012a,b, Whitney et al 2012.…”

Validated age and growth estimates for Carcharhinus obscurus in the northwestern Atlantic Ocean, with pre- and post management growth comparisons

“…However, the size range of the species was represented in all time periods, and geographic variation in parameters due to genetic effects is likely to be minimal; all samples used in this study were likely obtained from the same population of dusky sharks (Benavides et al 2011). Thus, while we acknowledge that there are limitations to these analyses in terms of sample sizes and frequency distributions, we also feel it would have been remiss not to test for density-dependant differences due to the exploitation history of this species.…”

“…The dusky shark is a common coastal pelagic carcharhinid that is widely distributed in the world's oceans (Compagno 1984) and considered a single, genetic stock in the northwest Atlantic and GOM (Benavides et al 2011). It is considered one of the most vulnerable species to overexploitation due to its combination of slow growth, late age at maturity (19-21 years), small litter sizes (3-12 embryos), and tri-annual gestation (Natanson et al 1995;Romine et al 2009).…”