Longshore movements and site fidelity of the iconic giant trevally Caranx ignobilis from southern Africa, determined using passive acoustic telemetry

Abstract

Giant trevally Caranx ignobilis are iconic, apex marine predators globally distributed in tropical to subtropical regions, where they are of high importance to ecosystems and fisheries. During summer, adults aggregate for spawning, making them vulnerable to overfishing. The world’s largest C. ignobilis aggregation has been recorded in southern Mozambique, but the year-round movements of these fish required further investigation. To assess movement patterns of C. ignobilis along the southern African coast, 36 adult fish were acoustically tagged and monitored along the east coast between Santa Maria in southern Mozambique and Port St Johns in the Eastern Cape, South Africa, for over 5 yr using an extensive passive acoustic receiver array. All acoustically tagged C. ignobilis were recorded in southern Mozambique between November and January each year (with minor exceptions), moving distances up to a maximum of 632 km in one direction. These movements were fast and direct, with maximum travelling speeds of up to 130 km d-1. When not migrating, South African-based fish showed consistent inter-annual fidelity to individual linear areas of space use equivalent to home ranges. Although the coastal length of these areas varied considerably between individuals, even the mean length (92 km) was greater than any previously recorded C. ignobilis home range, globally. Findings from this study, including the value of no-take marine protected areas, are of global ecological interest and have implications for local fisheries management. Protecting these vulnerable aggregations is of the utmost importance for the future of this species in southern African waters and elsewhere.