Abstract

Population genetics has been increasingly applied to study large sharks over the last decade. Whilst large shark species are often difficult to study with direct methods, improved knowledge is needed for both population management and conservation, especially for species vulnerable to anthropogenic and climatic impacts. The tiger shark, Galeocerdo cuvier, is an apex predator known to play important direct and indirect roles in tropical and subtropical marine ecosystems. While the global and Indo‐West Pacific population genetic structure of this species has recently been investigated, questions remain over population structure and demographic history within the western Indian (WIO) and within the western Pacific Oceans (WPO). To address the knowledge gap in tiger shark regional population structures, the genetic diversity of 286 individuals sampled in seven localities was investigated using 27 microsatellite loci and three mitochondrial genes (CR,COI, and cytb). A weak genetic differentiation was observed between the WIO and the WPO, suggesting high genetic connectivity. This result agrees with previous studies and highlights the importance of the pelagic behavior of this species to ensure gene flow. Using approximate Bayesian computation to couple information from both nuclear and mitochondrial markers, evidence of a recent bottleneck in the Holocene (2,000–3,000 years ago) was found, which is the most probable cause for the low genetic diversity observed. A contemporary effective population size as low as 111 [43,369] was estimated during the bottleneck. Together, these results indicate low genetic diversity that may reflect a vulnerable population sensitive to regional pressures. Conservation measures are thus needed to protect a species that is classified as Near Threatened.

Highlights

  • Study of large sharks, including the tiger shark Galeocerdo cuvier, the great white shark Carcharodon carcharias, and the whale shark Rhincodon typus, is challenging as these species spend substan‐ tial periods of their lifetime in open ocean waters

  • Samples were collected in the western Pacific from the northeast coast of Australia (Queensland, AUS2: n = 10) and in New Caledonia (NCA: n = 23; Figure 1)

  • Pairwise ΦST values for the control region (CR)‐COI‐cytb dataset were highly significant between: Reunion Island and AUS1 (West Australia); between AUS2 (East Australia) and New Caledonia; as well as be‐ tween South Africa and AUS1 (West Australia), AUS2 (East Australia), and New Caledonia (ΦST = [0.165, 0.387], all p < 0.001 after FDR correction; Table 5)

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Summary

| INTRODUCTION

Study of large sharks, including the tiger shark Galeocerdo cuvier, the great white shark Carcharodon carcharias, and the whale shark Rhincodon typus, is challenging as these species spend substan‐ tial periods of their lifetime in open ocean waters. Clarke et al (2006) estimated that ~400,000–500,000 tiger sharks are caught annually for the shark fin trade globally This species is currently targeted by shark control programmes in the Indo‐Pacific: South Africa (Cliff & Dudley, 1991; Dudley, 1997; Sumpton, Taylor, Gribble, McPherson, & Ham, 2011) and Australia (Holmes et al, 2012; Reid & Krogh, 1992; Simpfendorfer, 1992), and formerly in Hawaii (Wetherbee, Lowe, & Crow, 1994). CR sequences obtained by Bernard et al (2016) were used in conjunction with our data, to further investigate their proposed mitochondrial structure in these oceans

| MATERIALS AND METHODS
| Laboratory procedures
| DISCUSSION
Findings
| CONCLUSION
CONFLICT OF INTEREST
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