Habitat and movements of the swordfish Xiphias gladius in the southern Indian Ocean oligotrophic gyre and beyond: preliminary results of swordfish tagging experiments in Reunion Island

Habitat and migratory movements of swordfish in the Indian Ocean are still poorly known despite decades of research. Past tagging efforts were limited in time and space both due to a low survival rate of tagged swordfish and high cost of electronic tags. The limited number of swordfish tagged with conventional tags has provided a broad idea on the scale of horizontal displacements and tagging experiments using Pop-Up Satellite Archival Tags (PSATs) off South Africa have shown apparent site fidelity of swordfish in that area. Yet, the overall knowledge of the vertical habitat, dispersion rates and migratory patterns at the scale of the western Indian Ocean are still poorly known. Here we present results of swordfish tagging experiments using PSATs in the southwestern Indian Ocean that were carried out in the framework of EU-funded project PESCARUN where a total of 7 PSATs were deployed on swordfish between September 2021 and December 2022. We also used data from two swordfish tagged with PSATs in November 2012 and in December 2015 in the framework of the SWIOFP and PELICAN projects, respectively. All tagging operations took place within southwestern Indian Ocean oligotrophic gyre (in proximity with Reunion Island). To date, a total of 8 PSATs surfaced and reported data. During relatively short deployment periods (100 days max at liberty) swordfish performed large-scale movements throughout the western Indian Ocean reaching the Mozambique Channel in the west and equatorial waters in the north. Some individuals, however, showed ‘homing’ movements around Reunion Island circling back to the tagging place. All tagged individuals showed a similar vertical habitat occupation pattern: upper mixed layer/thermocline during the night and deeper hence colder mesopelagic layers around 500-900 m during daytime. Swordfish spent most of daytime (~80%) below 200 m depth suggesting that mesopelagic environment is its principal habitat in the western Indian Ocean. Diving behaviour along the northward migratory tracks showed habitat compression towards equatorial waters apparently rather driven by temperature limitations than dissolved O2 concentrations. Our results provide the first insights on the high rates of swordfish individuals’ dispersion at ocean-scale level.