MEPS prepress abstract   -  DOI: https://doi.org/10.3354/meps14786

ABSTRACT: The porbeagle shark (Lamna nasus) is a top marine predator in the North Atlantic that is vulnerable to anthropogenic stressors due to its life cycle characteristics and highly migratory behaviour across regions of commercial fisheries. Overcoming knowledge gaps on its biology, abundance and spatial ecology is crucial for underpinning effective conservation and management strategies for the species. In this study, we collected satellite tag data from 10 porbeagle sharks caught off the north coast of Ireland to study its migration behaviour and space-use in the northeast Atlantic. Analyses of data from pop-up satellite archival tags and Platform Terminal Telemetry tags collected between July 2010 and February 2014 (for deployments up to ~9 month) showed long-distance (1479-25707 km), seasonal migrations, with autumnal movements along the shelf-break to regions around Portugal, the Bay of Biscay and the Azores via the Mid-Atlantic ridge. Migrations to waters off Norway and the Faroe/Shetland Islands were also evident prior to these autumnal southward migrations. Following the onset of spring, some sharks returned northwards and there was evidence of site fidelity for shelf waters around the northern Irish coast and western Scotland and the Celtic Sea in summer. Porbeagles exhibited seasonal changes in vertical space-use as they traversed coastal, shelf-break and off-shelf habitats during their seasonal migrations, with deeper occupancy of the water column in winter than in summer. There was a distinct day-night pattern in porbeagle depth distribution during their off-shelf residency in winter, consistent with diel vertical migrations between deep waters in daytime and the surface layers at night. Nocturnal depth distribution was also closely associated with the monthly lunar cycle, with deeper residency and diving occurring consistently during periods of full moon. Our study shows that porbeagles occupy and traverse both the open ocean and coastal areas of high fishing activity and further highlights the challenge of assessing and managing the stock in the area because of their large-scale migratory behaviour.