Diving behaviour, movement patterns and population structure of blue sharks, Prionace glauca (L. 1758) in the North-east Atlantic

This project provided an exceptional opportunity to describe diving behaviour, movements and to characterise critical habitats of blue sharks in the North-eastern Atlantic using satellite and archival telemetry.  Tracked blue sharks displayed southward movements away from the tagging areas, exhibiting pronounced site fidelity to localised high-productivity frontal regions.  Blue sharks also displayed a high degree of variability in vertical movements. Shifts in diving behaviour were detected both within and between individuals, whereas behavioural phases were linked to the thermal structure of the water column in coastal areas, and to changes in prey distribution or type in offshore regions.  High resolution data showed that blue sharks shift between Brownian (in productive habitats) and Lévy (less productive waters) behaviours.  Vertical movements ranged from the surface to 1160 m, and water temperatures varied between 7.2-27.2°C.  Behavioural data was also crucial in determining the degree of spatial and temporal overlap, and thus vulnerability, between blue sharks and high-seas longliners.  Confirmed fishing mortality was ~11% with four tagged sharks caught by surface longliners.  Simulations showed that boats/sharks overlap was higher in winter and early spring, with the majority of simulated sharks (~88%) at risk at least one day year-1.  Our results indicate that, depending on which geographical regions are occupied at specific times, different segments of the blue shark population face differential risk from longlines.  Sequencing of mitochondrial DNA suggested an absence of spatial genetic differentiation throughout the North Atlantic, providing strong evidence that blue sharks comprise a single population in this region.