Resolving estimation of movement in a vertically migrating pelagic fish: Does GPS provide a solution?

Determining geo-positions from light data collected on broadbill swordfish has traditionally been problematic. Diving behaviour in this species is typically diel in nature, with the majority of time during the day spent at depths of approximately 600-800 m and the majority of time at night spent in waters typically less than 200 m. Descent into deep waters occurs at dawn and ascent into surface waters occurs at dusk. Diving behaviour such as this result in little light data being collected by archival tags deployed on this species and, as a consequence, calculated positions may be few and far between. This reduces the scale at which movement and habitat interaction can be inferred. Swordfish, however, do spend time right at the ocean surface at night and in some regions basking during the day also occurs. Periods of surface behaviour may provide the opportunity to determine position in this species utilising alternative technologies. In an effort to address this problem and assess the suitability of satellite based technologies for determining movement in swordfish, we developed a towed body tag incorporating Fastloc™ GPS technology that functions similarly to a pop-up satellite archival tag. Ten prototype tags were developed with deployments occurring on swordfish off the east coast of Australia during 2008. While tags were deployed on swordfish, GPS locations were recorded from eight of the 10 fish across 1.8-33.3% of days deployed (in comparison to 5.3-54.6% of positions determined from light). Utilisation of GPS technology in a configuration similar to a pop-up tag provides the potential for determining point estimates of position at higher accuracies than light data. By incorporating GPS positions in movement models with light-based positions, it also provides an ability to resolve movement at finer spatial scales than previously achievable. This in turn allows for the determining of habitats of importance, migratory corridors and the responses of individuals to spatial environmental variability at finer scales than previously possible and has broader application for marine wildlife management.