The basking shark, Cetorhinus maximus, has historically been a target of international fisheries, leading to well-documented declines in parts of its global distribution. Currently, the basking shark is listed as globally ‘Vulnerable’ and regionally ‘Endangered’ (North Pacific and Northeast Atlantic) on the IUCN Red List of Threatened Species, rendering the species an international conservation priority. Here, we assessed the global matrilineal genetic population structure and evolutionary history of the basking shark by completing the first whole mitochondrial genome sequence level survey of animals sampled from three globally widespread geographic regions: the western North Atlantic (n = 11), the eastern North Atlantic (n = 11), and within New Zealand territorial waters (n = 12). Despite the relatively large amount of sequence data assessed (~16,669 bp per individual), whole mitogenome analyses showed no evidence of population differentiation (ΦST = -0.047, P > 0.05) and very low nucleotide diversity (π = 0.0014 ± 0.000) across a global seascape. The absence of population structure across large distances and even between ocean basins is indicative of long-dispersal by this species, including an ability to cross known biogeographic barriers known to differentiate populations of other highly vagile pelagic fishes. Notably, evolutionary analyses of the mitogenome sequences revealed two globally sympatric but evolutionary divergent lineages, with a Bayesian framework estimated coalescence time of ~2.46 million years ago. Coalescent-based Bayesian skyline analysis uncovered subtle evidence of Pleistocene demographic flux for this species, including a potential decline in female effective population size. Thus, historical population changes may be responsible for the occurrence of the two highly divergent, yet sympatric lineages, as population declines may have resulted in the loss of intermediate haplotypes and resulted in an overall loss of genetic diversity. This work supports the recognition of basking sharks as a single matrilineal global population, and as such requires the application of a cooperative multiagency and international approach to fisheries management to conserve this highly vulnerable and ecologically unique species.

Large marine vertebrate species can exhibit vast movements, both horizontally and vertically, which challenges our ability to observe their behaviours at extended time-scales. There is a growing need to understand the intra- and inter-annual movements of mobile marine species of conservation concern in order to develop effective management strategies. The basking shark (Cetorhinus maximus) is the world's second largest fish species, however, a comprehensive understanding of this species’ ecology, biology and spatial behaviour in the north-east Atlantic is currently lacking. This thesis seeks to investigate the movement ecology of basking sharks using a suite of technologies to integrate biologging, biotelemetry, remotely sensed data, and ecological modelling techniques. I use satellite telemetry data from basking sharks tracked in 2012, 2013 and 2014 to quantify movements in coastal waters off the west coast of Scotland within the Sea of the Hebrides proposed MPA. Sharks exhibited seasonal residency to the proposed MPA, with three long-term tracked basking sharks demonstrating inter-annual site fidelity, returning to the same coastal waters in the year following tag deployment (Chapter 2). I reveal that sharks tracked into winter months exhibit one of three migration strategies spanning nine geo-political zones and the High Seas, demonstrating the need for multi-national cooperation in the management of this species across its range (Chapter 3). I examine the vertical space-use of basking sharks to improve an understanding of the processes that influence movements in all dimensions. Basking sharks exhibit seasonality in depth-use, conduct deep dives to over 1000 m, and alter their depth-use behaviour in order to remain within thermal niche of between 8 and 16 oC (Chapter 4). Finally, I combine contemporaneous data recorded by deployed satellite tags with remotely sensed environmental data to employ novel ecological modelling techniques to predict suitable habitat for basking sharks throughout the Atlantic Ocean (Chapter 5).