Connectivity, diversity, and demography of Lagenorhynchus dolphins of the North Atlantic

Abstract

The earth’s polar regions are facing an exceptional degree of environmental change in the Anthropocene, leading to an unprecedented rate of biodiversity loss. Cetaceans are important predators in these fragile ecosystems and are impacted by an array of multiple stressors, potentially impairing their fitness and long-term survival. Understanding to which extent cetacean populations are impacted requires comprehensive knowledge on their population dynamics. However, accessing elusive pelagic species, such as oceanic dolphins, proves to be a cost-intensive and high-risk endeavor. Consequently, large knowledge gaps remain in many species regarding their genetic connectivity, proportion of genetic variation, and demographic history. All of these aspects, however, are crucial for assessing the conservation status of present-day populations and help to make informed predictions on how future change may impact their survival. Due to dedicated monitoring of coastlines adjacent to the core habitat of understudied marine species over decades, opportunistic sampling of stranded cetaceans can provide a cost-effective opportunity to address these questions. Through international collaboration, samples collected by stranding networks in combination with other sampling approaches allow to obtain the samples needed to evaluate population dynamics of elusive species using state-of-the-art genomic approaches in an ecosystem-wide manner. The two extant species of the Lagenorhynchus genus of the North Atlantic Ocean, the Atlantic white-sided and the white-beaked dolphin, suffer from data deficiency due to their pelagic nature. A recent surge in interest from various stakeholders was driven by the prospective impact that climate change may have on these cold-water obligate species. Driven by recommendations to increase our understanding of Lagenorhynchus population dynamics and risk of impact from environmental stressors, this project aimed to gather tissue samples from stranding networks and other sources and implement high-throughput Next Generation Sequencing to address these components from a genetic perspective. As a result, this thesis provides novel insights into Lagenorhynchus biology and conservation, which are communicated through engagement with policymakers and the publication of scientific results. In the introduction of this thesis, I will lay out the early and recent advances in genetics and genomics to study natural populations globally and across species through time. With a focus on how genetic techniques have benefited cetacean conservation, I will then highlight the ongoing initiatives undertaken to understand and protect cetacean populations worldwide and in the North Atlantic specifically. The two target species and the importance of stranding archives and other sampling approaches for cetacean conservation are described to provide a context for the methodological approach of this thesis. Chapters 2 and 3 focus on each species separately, describing their genetic connectivity, diversity, relatedness, and gene flow informed by reduced representation sequencing. The results illustrate the stark contrast in population structure the two species display on a range-wide scale, reflecting their ecological differences and emphasising recommendations for conservation. Chapter 4 takes a comparative approach using whole-genome sequences to investigate patterns of genome-wide diversity and inbreeding, and to reconstruct the demographic histories of the species. The results put the previously assessed population structure in a wider evolutionary context, and allow for some predictions of future responses to global climate change aided by a species distribution modelling approach. In the final chapter of the thesis, all results are put into a context of conservation genomics on a broader scale and in cetacean populations specifically. The impact these results have on the conservation of North Atlantic Lagenorhynchus dolphins is being presented and putative impact is discussed. Finally, I will highlight pressing needs for future research to further understand Lagenorhynchus populations and assess their conservation status in light of increasing pressure through various anthropogenic stressors.