Migration and space use by porbeagle sharks Lamna nasus in the northeast Atlantic

The porbeagle shark Lamna nasus, a top predator in the North Atlantic, is vulnerable to anthropogenic stressors due to its life history characteristics. Understanding its biology, abundance and spatial ecology is crucial for underpinning effective conservation and management strategies. We collected satellite tag data from 10 porbeagle sharks caught off the north coast of Ireland to study migration behaviour and space use. Data from pop-up satellite archival tags and platform terminal telemetry tags collected between July 2010 and February 2014 (for deployments up to ~9 mo) 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. In 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 various habitats during migration, 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 closely associated with the lunar cycle, with deeper residency/diving occurring during periods of full moon. Porbeagles occupied and traversed both the open ocean and coastal areas of high fishing activity, highlighting the challenge of managing this stock because of large-scale migratory behaviour.