Genome-wide analysis reveals striking lack of genetic differentiation over long distances for the Antarctic toothfish Dissostichus mawsoni: High genetic connectivity or shared spawning grounds?

The giant Antarctic toothfish Dissostichus mawsoni is one of the most economically valued fishery resources from the Southern Ocean. However, the distinctive biological characteristics and life history of this species, including large body size, long life span, late sexual maturity, and specialization to freezing polar waters, make it particularly vulnerable to overfishing and global climate change. It is therefore of fundamental importance to administer sound management and conservation actions to avoid depletion of this unique species. The crucial issue of whether this species consists of distinct biological units or stocks, which is needed to inform appropriate management and conservation, is currently unresolved. We report here the first RAD-seq analysis of Antarctic toothfish populations, a powerful approach of genome-wide discovery of SNP loci, to assess genetic differentiation between the geographically distant individuals from CCAMLAR Subarea 48.1 locations (South Shetland Islands and West Antarctic Peninsula coasts) and Subarea 88.1 (Ross Sea and McMurdo Sound). We recovered 19,611 SNPs belonging to 13,251 widely shared loci. Comprehensive population structure analyses unambiguously indicated absence of any significant level of population differentiation, thus the null hypothesis of panmixia cannot be rejected. Together with currently known life history traits of D. mawsoni, we suggest this striking lack of genomic differentiation likely reflects high degrees of contemporary and/or historical gene flow rather than shared spawning grounds. Complementary analyses, particularly otolith microchemistry, would contribute useful inference of natal origins.