Population genetics of the school shark (Galeorhinus galeus) in New Zealand, Australian and Chilean waters
Authors:   , Hernández Muñoz Sebastián Ignacio 
Published: November 2021

<p>The school shark (Galeorhinus galeus) is a coastal bentho-pelagic species that is highly migratory and has a widespread distribution in temperate waters. This species matures late, has a relatively low fecundity and is slow growing, which makes it vulnerable to overfishing. They are commercially fished throughout their distribution, and some global stocks have been under pressure because of poor management. In Australia, longline and gillnet fisheries targeted pregnant females and juveniles around Victorian and Tasmanian nursery grounds, resulting in loss of historical inshore nursery habitat. School shark tagging programmes have reported migration between Australian and New Zealand stocks, but preliminary genetic studies have suggested that there are slight genetic differences between the stocks. Currently, the Australian and New Zealand school shark fisheries are assessed and managed as separate stocks. However, the question of whether this species is comprised of a single population or multiple sub-populations in the South Pacific remains unresolved. Given the commercial importance of the school shark fisheries and the concern about stock levels on the regional and trans-Tasman scales, knowledge of stock structure is essential for effective management. The aim of this thesis research was to determine the levels of genetic diversity and population structure of G. galeus in New Zealand and Australia, and compare these to a population in Chile, using mitochondrial DNA (mtDNA) sequencing and microsatellite DNA markers.  The DNA sequence of an 893 base pair region of the mtDNA control region (CR) was determined using 475 school shark samples and nine microsatellite DNA loci were genotyped in 239 individuals. Analyses of the data revealed strong evidence of genetic differentiation between G. galeus populations in Australasia and Chile, suggesting restricted gene flow among populations in the western and eastern areas of the Pacific Ocean. The FST values ranged from 0.188 to 0.300 for CR mtDNA, and 0.195 to 0.247 for microsatellite DNA in G. galeus. However, there was no evidence of stock differentiation among New Zealand/Australian sample sites for either mtDNA or microsatellite DNA data. These results support the model of a single panmictic stock across the Tasman Sea. The similarity of the results obtained from the maternally inherited mtDNA and biparental inherited microsatellite loci did not support the suggestion of sex-biased dispersal of G. galeus in the New Zealand/Australia region and it was concluded that females and males had similar patterns of dispersal.  Sharks can be either monogamous or polygamous, which is important when considering stock assessments and harvesting models. Multiple paternity has been reported in several shark species, however, the number of sires per litter varies considerably among species. An investigation of multiple paternity (MP) was conducted in G. galeus by assessing the levels of relatedness within progeny arrays using six polymorphic microsatellite DNA markers. Five “families” (mother and litters) were sampled from the North Island of New Zealand and a parentage analysis was conducted. The minimum number of males contributing to each progeny array was estimated by identifying the putative paternal alleles by allele counting and reconstructing multilocus genotypes method. The analysis showed the occurrence of genetic polyandry in G. galeus; two of five litters showing multiple sires involved in the progeny arrays (40%). The minimum number of sires per litter ranged from one to four. Although MP was only detected in two litters, this finding is consistent with the known reproductive characteristics of G. galeus. It can potentially store sperm for long periods of time and has a specific mating season when males and females typically mix on the edge of the continental shelf. Detecting MP within a litter has highlighted the importance of the post-copulatory selective processes in the G. galeus mating system, and this has implications for the management and conservation of genetic diversity.</p>