Acoustic telemetry reveals multi-seasonal spatiotemporal dynamics of a giant trevally Caranx ignobilis aggregation

Predictable fish aggregations of commercially valuable species are particularly susceptible to overexploitation. Giant trevally Caranx ignobilis are ecologically important top predatory fish targeted in both recreational and commercial fisheries; however, little is known about their aggregation dynamics or susceptibility to overexploitation. This study employed acoustic telemetry to investigate the temporal and spatial characteristics of one of the largest known aggregations of this species over a 3 yr period in a marine protected area in Mozambique, West Indian Ocean. Tagged fish were monitored for between 386 and 1176 d, during which they exhibited distinct aggregative periods in austral spring and summer. The detection frequency of fish at the aggregation site was significantly associated with rising mean sea temperatures and the full moon period. Whilst at the aggregation site, fish exhibited clear patterns of diel periodicity, with maximum hourly detections recorded during midday. All fish exhibited periods of absence from the aggregation site between summer seasons, in which time many of them ranged across an international border for distances of between 11 and 633 km before returning to aggregate at the same site in the following season. These results confirm that the studied giant trevally aggregation is temporally and spatially predictable, and consistent with a transient site-specific fish spawning aggregation. The spatiotemporal predictability of the aggregation highlights the need for the effective management of this transboundary population of giant trevally, for which this study has provided conservation management recommendations.