Weak population structure of the Spot‐tail shark Carcharhinus sorrah and the Blacktip shark C. limbatus along the coasts of the Arabian Peninsula, Pakistan, and South Africa

Abstract

The increase in demand for shark meat and fins has placed shark populations worldwide under high fishing pressure. In the Arabian region, the spot‐tail shark Carcharhinus sorrah and the Blacktip shark Carcharhinus limbatus are among the most exploited species. In this study, we investigated the population genetic structure of C. sorrah (n = 327) along the coasts of the Arabian Peninsula and of C. limbatus (n = 525) along the Arabian coasts, Pakistan, and KwaZulu‐Natal, South Africa, using microsatellite markers (15 and 11 loci, respectively). Our findings support weak population structure in both species. Carcharhinus sorrah exhibited a fine structure, subdividing the area into three groups. The first group comprises all samples from Bahrain, the second from the UAE and Yemen, and the third from Oman. Similarly, C. limbatus exhibited population subdivision into three groups. The first group, comprising samples from Bahrain and Kuwait, was highly differentiated from the second and third groups, comprising samples from Oman, Pakistan, the UAE, and Yemen; and South Africa and the Saudi Arabian Red Sea, respectively. Population divisions were supported by pairwise FST values and discriminant analysis of principal components (DAPC), but not by STRUCTURE. We suggest that the mostly low but significant pairwise FST values in our study are suggestive of fine population structure, which is possibly attributable to behavioral traits such as residency in C. sorrah and site fidelity and philopatry in C. limbatus. However, for all samples obtained from the northern parts of the Gulf (Bahrain and/or Kuwait) in both species, the higher but significant pairwise FST values could possibly be a result of founder effects during the Tethys Sea closure. Based on DAPC and FST results, we suggest each population to be treated as independent management unit, as conservation concerns emerge.

Killer whales (Orcinus orca) in Iceland show weak genetic structure among diverse isotopic signatures and observed movement patterns

“…The F ST values in our study are of similar magnitude to those in several mammal populations, assessed at the level of social groups with philopatry and gene flow among groups (Storz, 1999). Low genetic divergence has been commonly found in studies of marine species (e.g., Almojil et al, 2018;Vignaud et al, 2014;Ward, Woodwark, & Skibinski, 1994).…”

“…The F ST values in our study are of similar magnitude to those in several mammal populations, assessed at the level of social groups with philopatry and gene flow among groups (Storz, ). Low genetic divergence has been commonly found in studies of marine species (e.g., Almojil et al, ; Vignaud et al, ; Ward, Woodwark, & Skibinski, ). There was also extremely low mtDNA haplotype variation in our dataset, consistent with low worldwide mtDNA diversity patterns in killer whales (e.g., Hoelzel et al, ; Morin et al, ; Moura, Janse van Rensburg, et al, ).…”

“…Furthermore, STRUCTURE can fail to identify genetic structure when: (a) allele frequencies vary gradually across a region (e.g., when there is isolation-by-distance) (Pritchard, Wen, & Falush, 2009); (b) sample size of genetic groups is limited or uneven (Kalinowski, 2011;Puechmaille, 2016;Waples & Gaggiotti, 2006); (c) gene flow is relatively high (Waples & Gaggiotti, 2006); and (d) mutation is low and genetic differentiation is limited (Almojil, Cliff, & Spaet, 2018;Barr et al, 2008;Latch, Dharmarajan, Glaubitz, & Rhodes, 2006;Waples & Gaggiotti, 2006). Contrarily, in cases of weak genetic differentiation, principal component analysis has proven to be a sensitive tool in detecting fine-scale structure (Novembre et al, 2008;O'Connor et al, 2015); for example, using DAPC, Almojil et al (2018) found biologically meaningful genetic clusters where STRUCTURE failed to find any (as in this study). Therefore, we believe that for our dataset, DAPC was a more sensitive approach to accurately investigate finescale genetic differences among samples.…”