Abstract
Knowledge about the demographic histories of natural populations helps to evaluate their conservation status, and potential impacts of natural and anthropogenic pressures. In particular, estimates of effective population size obtained through molecular data can provide useful information to guide management decisions for vulnerable populations. The spotted ragged-tooth shark, Carcharias taurus (also known as the sandtiger or grey nurse shark), is widely distributed in warm-temperate and subtropical waters, but has suffered severe population declines across much of its range as a result of overexploitation. Here, we used multilocus genotype data to investigate the demographic history of the South African C. taurus population. Using approximate Bayesian computation and likelihood-based importance sampling, we found that the population underwent a historical range expansion that may have been linked to climatic changes during the late Pleistocene. There was no evidence for a recent anthropogenic decline. Together with census data suggesting a stable population, these results support the idea that fishing pressure and other threats have so far not been detrimental to the local C. taurus population. The results reported here indicate that South Africa could possibly harbour the last remaining, relatively pristine population of this widespread but vulnerable top predator.
Highlights
The size of a population is a determining factor in how evolutionary and demographic processes affect its longterm survival
To gain a better understanding of whether population trends inferred from catch data present an accurate picture of the health of this species, we used genetic data to explore the demographic history of the South African C. taurus population
The principal coordinate analysis (PCoA) further indicated genetic homogeneity, with the first two axes only explaining 4.34% and 3.63% of the variation in the data, and overlap of groups from different sampling sites being evident (Supplementary Fig. S1). These results indicate that Carcharias taurus in South Africa comprises a single population and justify the inclusion of all available data in the subsequent demographic analyses
Summary
The size of a population is a determining factor in how evolutionary and demographic processes affect its longterm survival. Several studies have attempted to establish the ratio of Ne and Nc, with the expectation that the estimation of one parameter would allow the inference of the o ther[10] This is an active area of research, ratios remain uncertain, as they are influenced by a variety of factors which interact in reducing Ne. Reported average Ne/Nc ratios across a range of species from different taxa range from approximately 0.1 to 0.511–13. It was estimated that an average of 6,800 juvenile and 16,700 adult C. taurus inhabit the South African coast This modelling approach was applied to a shark species for the first time, and some uncertainties remain around the parameters of the m odel[36]. This represents the first genetic assessment of its kind for this population and may help to direct future management decisions
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