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MEPS prepress abstract   -  DOI: https://doi.org/10.3354/meps14413

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

Ryan Daly, Stephanie K. Venables, Toby D. Rogers, John D. Filmalter, Tessa N. Hempson, Taryn Murray, Nigel E. Hussey, Isabel Silva, Marcos A. M. Pereira, Bruce Q. Mann, Bilardo A. S. Nharreluga, Paul D. Cowley

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ABSTRACT: 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 four 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 four years (2018-2022) on an acoustic receiver network of 350 receivers. During this period, 63% of bull sharks (n=19), 87% of blacktips (n=13), 94% of tiger (n=16) and 25% of grey reef sharks (n=3) tagged, undertook transboundary movements. The frequency of mean transboundary movements ranged between 1.3 (SD±1.5; grey reef sharks) and 81 (SD±35.6; tiger sharks) per year. 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 emphasizes the need for collaborative transboundary cooperation between the two countries and the alignment of regional management plans and interventions to address declining shark populations in this region of the Western Indian Ocean.