A whale's tale of mtDNA diversity and differentiation : the Antarctic blue whale

Abstract

Twentieth century commercial whaling drastically reduced the abundance of great whale populations in the Southern Ocean. Exploitation began on the south Atlantic island of South Georgia, where catch records account for over 175,000 whales killed. Modern whaling within the Southern Ocean depleted populations rapidly, and by 1966, hunting blue whales south of 40°S was prohibited by the International Whaling Commission (IWC). After 40 years of protection, this species has shown little recovery. A current abundance estimate of 2,280 (CV=0.036) individuals from sighting data (1991/92-2003/04) represents less than 1% the original abundance. With such an intensive demographic 'bottleneck,' it is likely that genetic diversity has been lost from some or all components of the Southern Ocean population. Here I describe historical and contemporary Antarctic blue whale mtDNA diversity and report the first circumpolar analyses of contemporary population structure. In Chapter 2, historical mtDNA diversity is described from whale bones collected from the first Southern Hemisphere whaling stations established in 1904 on the island of South Georgia. A total of 281 whale bones were representative of three prominent species hunted in South Georgian waters. Using ancient DNA methods and sequencing of the mtDNA control region, bone samples were first identified to species, identifying 153 humpback, 49 fin, 18 blue, 2 sei, 1 southern right whale and 1 elephant seal. Within each of the three prominent historic species populations, mtDNA haplotypes were described resulting in 64 humpback, 34 fin, and 16 blue whale haplotypes. Haplotype and nucleotide diversity within each of the three historic species populations ranged from 0.980-0.987 and 1.87-3.16%, respectively. In chapter 3, I update the previous estimate of contemporary Antarctic blue whale mtDNA diversity with biopsy samples of living whales collected during research cruises conducted with IWC oversight from 1990-2009 (n=218) for comparison to historical blue whale mtDNA diversity. After the removal of replicate samples based on 15 microsatellite loci, the dataset described 167 individuals. This dataset was combined with additional published Antarctic blue whale mtDNA control region sequences (LeDuc et al. 2007; n=20) to represent the most comprehensive dataset available for Antarctic blue whale mtDNA diversity (n=187). A high haplotype diversity was described within this contemoporary population (0.968). With this dataset, I report the first evidence of population structure within the IWC Southern Ocean management Areas I-VI through an analysis of genetic differentiation. The identification of recaptures within the dataset through microsatellite genotyping, allows for the first inference of movement of six individuals with the Southern Ocean since the end of the Discovery marking program 50 years ago. In the final chapter of this thesis, I explore the impact of the 20th century commercial whaling industry on the Antarctic blue whale population through a comparison of historical and contemporary Antarctic blue whale mtDNA diversity. The comparison showed that only 6 of the 16 haplotypes from the South Georgian population were found in contemporary worldwide blue whale populations, indicating a potential loss of mtDNA lineages. The loss of mtDNA haplotypes sugges two hypotheses; either a low predicted loss of widespread Antarctic blue whale mtDNA diversity or the loss of a South Georgia local Antarctic blue whale population driven to commercial extinction. The impact of commercial whaling is also assessed through a prediction of the minimum number of maternal lineages, or haplotypes, to have survived the exploitation bottleneck. The number of 51 mtDNA haplotypes identified within the contemporary Antarctic blue whale population is used to update the estimate of haplotypes within the unsampled contemporary population. We predict 69 mtDNA lineages within the contemporary population from the current abundance estimate of 2,280 (1,160-4,500) individuals (Branch 2008). This prediction will increase the lower bound of population abundance used in population dynamic modeling and may reduce an upward bias in population increase estimates used to assess the recovery of this species.