The Gulf of Carpentaria, Coral, Arafura and Timor seas of the Northern Territory and far North Queensland have unique billfish fisheries with impressive seasonal aggregations occurring during the Austral spring build-up. No electronic tags have been reported from central northern Australia and only a handful of studies exist from the north-east coast. Between October 2022 and November 2024 we deployed 30 Pop-off Archival Satellite Tags (PSATs) tags (28 sailfish, two black marlin) and 39 acoustic V16 transmitters (36 sailfish, three black marlin), spanning internal and external attachment, to collect movement data, compare tagging methods and assess the feasibility of acoustic telemetry in billfish. Eleven PSATs returned data, with deployments ranging from two to 339 days.
Sailfish were surface-associated and exhibited diel vertical movement that was modulated by moonlight, with fish diving deeper on brighter nights. Maximum daily depth also varied with tag-measured sea surface temperature and month. Fish tagged in the Gulf of Carpentaria during the Austral spring dispersed as far as the Banda Sea by autumn, changing their dive profiles as they went. Additionally, double-tagging sailfish with both PSATs and acoustic transmitters allowed light-based geolocation error to be measured directly against known detections for one individual, an assessment rarely possible in billfish. Acoustic telemetry proved surprisingly effective, with 25 of 39 tags detected, the longest 874 days after tagging. This study reports the first electronic tagging from central northern Australia and identifies previously unobserved migration routes.