Estimating Connectivity through Larval Dispersal in the Western Indian Ocean

The health and functioning of coral reef ecosystems in the Western Indian Ocean are in decline due to anthropogenic stress, and the rate of decline is set to accelerate. Marine reserves have become important tools in mitigating these pressures, and one of the most critical factors in determining their spatial design is the degree of connectivity among populations. However, the spatial scale and strength at which populations are potentially connected by dispersal remain uncertain. In this paper, the potential connectivity of corals was assessed through a Lagrangian particle-tracking algorithm coupled offline with an ocean circulation numerical model. Connectivity metrics and graph theory were used to analyze the networks and highlight those reefs that are critical for maintaining the connectivity of corals across the region. Western Indian Ocean coral reefs formed about 13 clusters for larvae with short pelagic larval duration (PLD; 5 days) but grouped into stable larger clusters at 15, 30, and 60 days PLDs, with strong westward dispersal from the northern tip of Madagascar and Comoro Islands toward the East African coast. Reefs at Mafia-Latham Island (Tanzania), Angoche-Nacala and Pemba-Mucufi (Mozambique), Anjouan and Ngazidja (Comoros), and Glorieuses (France) were the most significant for multigenerational connectivity, as measured by betweenness centrality. The small islands Agalega and Tromelin were completely disconnected from other sites at all PLDs. These findings identify priority conservation areas and actions to help support coral reef resilience through a connected network of marine protected areas. Plain Language Summary Given that the health of coral reefs have declined due to a combination of global and local stressors, managing this ecosystem and associated resources is crucial. The establishment of marine protected areas is one widely used conservation instrument. This study assesses the potential connectivity of coral reefs within the Western Indian Ocean region using a biophysical model (i.e., a Lagrangian particle-tracking algorithm coupled with an ocean circulation numerical model). The results show that coral reefs in the Western Indian Ocean form a relatively well connected network, with reefs at Mafia-Latham Island (Tanzania), Angoche-Nacala and Pemba-Mucufi (Mozambique), Anjouan and Ngazidja (Comoros), and Glorieuses (France) being the most significant for multigenerational connectivity. This indicates the imperative need to include connectivity information when considering setting up marine protected areas.