Modelling coral reef connectivity in the Western Indian Ocean under climate change

Coral reefs worldwide arefacing more risks than ever before.The Intergovernmental Panel on Climate Change(IPCC)published that global ocean warming is strongest at the surface and the Indian Ocean surface temperatures have increased at a rate of0.11 C per decade, fasterthan for the Atlantic and Pacific Oceans.(Hoegh-Guldberg et al., 2014; IPCC, 2018; van Vuuren et al., 2011)The Indian Ocean warming of 0.65 C compared to 1950 -2009 exceeds the other oceans as well(Hoegh-Guldberg et al., 2014). Average live coral cover has decreased by 50 -75 % almost everywherein the last 30 -40 years(Bruno et al., 2019).This situation is predicted to get worse with future mass bleaching events (Hughes et al., 2017). A third of reef-forming coral species are considered threatened and another third as near threatened in the IUCN Red List (Bridge et al., 2020).These taxa are critical for reef resilience and complexity which form the base of healthy fish communities(Eakin et al., 2019). Coastal protection and food security under global change depend on efficient conservation management that accounts for genetic connectivity of corals under present conditions and future climate change scenarios (Graham et al., 2015; Hughes et al., 2017).Current models of coral or reef fish distribution under climate change often do not include larval dispersal which is a critical factor that influences the trajectory of reefs after a disturbance event such as bleaching(Graham et al., 2015; Magris et al., 2016).The Indian Ocean is especially heavily impacted by climate change and reefs in the Western region are essential resourcesfor some of the world's poorest communities.Currently, only three key connected reefs are under protectionand conservation focus on the other highly connectedreefs is requiredin order for them to act as stepping stones for larval and gene dispersal(Gamoyo et al., 2019). However,knowledge about coral connectivity and reef resilience under future climate scenarios in this geographical region is limited despite increasing threats and anthropogenic pressure. With further climate impacts it is vital to understand how present circulation patterns and coral and fish population dynamics might change and how this affects reef resilience and food security. Marine protected areas (MPAs)emerged as one of the key measures to support coral reefs but their success depends on how connected they are on large scales by movement of fish or larval dispersal (Crochelet et al., 2016). One of the world's largest no-take marine reserves is the British Indian Ocean Territory(BIOT) MPAin the centre of the Indian Ocean which was designated by the UK in 2010. Since the 1970s, the archipelago is uninhabited except for a US military base on the largest atoll,Diego Garcia,and thus, has no fishing pressure besides some illegal activities. Despite its protection and remote location with minimal anthropogenic impacts the reefs in the archipelago endured severe bleaching and mortality with coral cover decreasingfrom 30% to 12 %between 2012 and 2016. Acropora, one of the key ecosystem architects, showed an 86% declineand was replaced by Porites after the bleaching event. (Head et al., 2019) With its central Indian Ocean location halfway between Africa and Indonesia this large MPA could act as an important stepping stone linking the Western and Eastern Indian Ocean under global change (Robinson et al., 2017). My research aims to model coral reef connectivity under present conditions and the IPCC climate scenarios in the Western Indian Oceanand the BIOT.Knowledge on connectivity of reef-building corals can underpin observational studies and improve predictions of species distributions under climate change scenarios (Wood et al., 2014).