Towards better management of Australia's shark fishery: genetic analyses reveal unexpected ratios of cryptic blacktip species Carcharhinus tilstoni and C. limbatus

Abstract

The common blacktip shark (Carcharhinus limbatus) and the Australian blacktip shark (C. tilstoni) are morphologically similar species that co-occur in subtropical and tropical Australia. In striking contrast to what has been previously reported, we demonstrate that the common blacktip shark is not rare in northern Australia but occurs in approximately equal frequencies with the Australian blacktip shark. Management of shark resources in northern Australia needs to take account of this new information. Species identification was performed using nucleotide sequences of the control, NADH dehydrogenase subunit 4 (ND4) and cytochrome oxidase I (COI) regions in the mitochondrial genome. The proportion of overall genetic variation (FST) between the two species was small (0.042, P < 0.01) based on allele frequencies at five microsatellite loci. We confirm that a third blacktip species (C. amblyrhynchoides, graceful shark) is closely related to C. tilstoni and C. limbatus and can be distinguished from them on the basis of mtDNA sequences from two gene regions. The Australian blacktip shark (C. tilstoni) was not encountered among 20 samples from central Indonesia that were later confirmed to be common blacktip and graceful sharks. Fisheries regulators urgently need new information on life history, population structure and morphological characters for species identification of blacktip shark species in Australia.

Rediscovery and redescription of the smoothtooth blacktip shark, Carcharhinus leiodon (Carcharhinidae), from Kuwait, with notes on its possible conservation status

“…The observation that there are greater differences in the COI barcode sequence among populations of the same nominal species (C. amblyrhynchoides) than there are between individuals of different species (C. leiodon and C. amblyrhynchoides) could give pause to those who would base species description on sequence data without a thorough sampling of all of the geographic variation circumscribed by a given species. Our data are consistent with the close relationship of C. amblyrhynchoides, C. limbatus and C. tilstoni suggested by Ward et al (2008) and recently confirmed by Ovenden et al (2010), who in addition to COI, used sequences from the control region and ND4 regions of the mitochondrial genome. However, the exact inferred relationships differ among studies.…”

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

“…PCR was then used to amplify the mitochondrial ND4 gene from all DNA extractions. This gene was selected for analysis following Dudgeon et al (2009) and Ovenden et al (2010), who demonstrated the ND4 gene to be the most polymorphic among a range of mtDNA markers (including the control region) in species related to those under study here. PCR reactions were carried out in 50 µl volumes containing 1 µl of DNA template, 1× GoTaq Colourless reaction buffer (consisting of 1.5 mM MgCl 2 and 200 µM deoxynucleoside triphosphates [dNTPs]) (Promega), 0.5 µl of RNase (1 mg ml −1 ), and 0.5 µM of each of the primers ND4 (5' CAC CTA TGA CTA CCA AAA GCT CAT GTA GAA GC) (Arèvalo et al 1994) and H12293-LEU (5' TTG CAC CAA GAG TTT TTG GTT CCT AAG ACC) (Inoue et al 2001).…”

More analytical bite in estimating targets for shark harvest

“…Given that the degree of spatial and temporal overlap and hybridisation of C. tilstoni and C. limbatus remains unclear (Ovenden et al 2010, Morgan et al 2011, Tillett et al 2012, and that these 2 species demonstrate differential susceptibility to harvest, managers must attempt to estimate their relative contributions in catches. Our models indicate that the more C. limbatus is represented in catches, the lower the harvest rate the combined population can tolerate.…”