Movement patterns of the iconic giant trevally Caranx ignobilis from southern Africa, determined using tag-recapture data

Giant trevally Caranx ignobilis, the largest species in the genus Caranx, are global icons as apex marine predators and contribute substantially to global fisheries. Understanding their movement patterns is critical for effective management, especially in the case of transboundary stocks. In southern Africa, there is limited published movement data for the species, apart from studies at the world's largest recorded aggregation of C. ignobilis globally, in southern Mozambique. Therefore, the broad aim of this study was to describe the coastal movements of C. ignobilis from southern Africa using data from South Africa's longest-running citizen science tag-recapture project. A total of 3 729 fish were dart tagged between 1984 and 2020, producing 144 recaptures. While 74% of recaptures were recorded < 1 km from the tagging location, long-distance movements of up to 419 km were also recorded (mean = 15 km). Although adults moved significantly (p < 0.01) greater distances than juveniles, they also displayed high levels of site fidelity. Seasonal trends included evidence of a summer migration from South Africa to southern Mozambique; but surprisingly, no transboundary movements were recorded. The data suggest long-term site fidelity with interspersed return migrations, likely for spawning. However, for the purposes of management, further research would be required to determine the proportion of the population that undertakes transboundary migration, rendering them potentially vulnerable to overfishing in foreign waters.