A COMPARISON OF DEMOGRAPHIC AND GENETIC ESTIMATES OF POPULATION SIZE IN BONNETHEADS (Sphyrna tiburo) AND SANDBAR SHARKS (Carcharhinus plumbeus)

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DeHart, Hayley
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Estimating population sizes using multiple estimates, as well as multiple species, is critical to understanding true population sizes and life history traits that skew these estimates. The current study aimed to estimate census population size (Nc) and compare the estimates to the genetically determined effective population size (Ne) in two shark species: bonnetheads (Sphyrna tiburo) and sandbar sharks (Carcharhinus plumbeus). Census size was estimated using data collected from the South Carolina Department of Natural Resources Cooperative Atlantic States Shark Pupping and Nursery Survey (COASTSPAN) and using Robust-Design in the program MARK. Cross-species gene capture was used to obtain 1077 single-copy exons and flanking intronic regions in Atlantic and alternate population samples. Linkage Disequilibrium models of single fragment SNPs were used to estimate a local Effective number of Breeders (Nb), and the unfolded Site Frequency Spectrum (SFS) was examined to determine past genetic demography and contemporary Ne. Local Nc estimates were best modeled with temporary emigration and ranged from 1700 and 3400 individuals of C. plumbeus and S. tiburo, respectively. There was a higher probability of emigration in S. tiburo than in C. plumbeus, which is consistent with prior research. Estimates of Nb using LD were estimated at 694 and 1119 for C. plumbeus and S. tiburo, respectively. Ratios of Ne/Nc were similar for both species at 0.3 and similar to previous studies. The SFS indicated positive growth signatures in C. plumbeus Atlantic and Pacific samples and a contemporary Ne of 7692 in the Atlantic. A more stable Atlantic population of S. tiburo and a large Gulf of Mexico population expansion was modeled in S. tiburo, with a contemporary Ne of 5905 in the Atlantic. These results highlight the importance of model selection in determining demographic parameters and provide promising results of present and past demography in these ecologically important and newly threatened organisms.
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