Persistent transboundary movements of threatened sharks highlight the importance of cooperative management for effective conservation

Migratory sharks play a key ecological role through movements within and among marine ecosystems, yet many populations are declining. Addressing the decline is especially challenging for wide-ranging species, as they may undertake movements between countries with disparate conservation priorities. To investigate the transboundary migrations of threatened sharks between neighbouring South Africa and Mozambique, we tracked 4 commonly occurring carcharhinid species (bull, blacktip, tiger and grey reef sharks). A total of 102 individuals were fitted with long-life acoustic transmitters and monitored for 4 yr (2018-2022) on an acoustic receiver network of 350 receivers. During this period, 63% of tagged bull sharks (n = 19), 87% of blacktips (n = 13), 94% of tiger (n = 16) and 25% of grey reef sharks (n = 3) undertook transboundary movements. The frequency of mean transboundary movements per year ranged between 1.3 +/- 1.5 (SD) for grey reef sharks and 81 +/- 35.6 for tiger sharks. Blacktip, bull and tiger sharks all undertook long-distance transboundary migrations ranging from 980 to 2256 km. These data confirm high connectivity between neighbouring countries by threatened sharks undertaking persistent transboundary movements. This study emphasizes the need for collaborative transboundary cooperation between the 2 countries and the alignment of regional management plans and interventions to address declining shark populations in this region of the Western Indian Ocean.