Swordfish satellite tagging studies off eastern North America:
movements, migrations and population structure.
John Neilson, PhD
St Andrews Biological Station, Department of Fisheries and Oceans Canada
In a three-year investigation of Atlantic swordfish ( Xiphias gladius) movements and migrations in the North Atlantic , we present results from the first two years of the study. The long-distance horizontal movements of swordfish tagged in Canadian waters are described, using information obtained from pop-up satellite archival tags. Some of the satellite tags remained attached to the fish for up to 411 days, among the longest periods of attachment of pop-up satellite tags reported for any fish species. The results to date challenge the assumption employed in current stock assessments that swordfish move freely from the western North Atlantic to the eastern North Atlantic , as no such movement has been found. The results also demonstrate a consistent pattern of movement with residence in temperate waters from June to October, followed by migration to the south into the Caribbean Sea , with fish remaining there until April. Tagged swordfish returned to temperate waters by June, with evidence of precise homing to feeding areas. The available information supports the existence of a metapopulation (i.e., a population of populations (Levins 1969)). The current management unit used for the assessment of North Atlantic swordfish does not yet recognize this level of population diversity. There is scope for refinement of the existing management approach, and inclusion of measures to protect population components. Finally, we describe the pattern of diel vertical movements of swordfish, and how they change throughout the year.
Swordfish basking at the surface (www.pier.org)
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