Contents lists available at ScienceDirect Marine Pollution Bulletin journal homepage: www.elsevier.com/locate/marpolbul Healthy and diverse coral reefs in Djibouti – A resilient reef system or few anthropogenic threats? B. Cowburn a,b, ⁎ , M.A. Samoilys a , K. Osuka a , R. Klaus b,c , C. Newman b , M. Gudka a , D. Obura a a CORDIO East Africa, 9 Kibaki Flats, Kenyatta Beach, Bamburi Beach, P.O.BOX 10135, Mombasa 80101, Kenya b Cousteau Society, 40 rue des Renaudes, 75017 Paris, France c Senckenberg Research Institute and Museum of Nature Frankfurt, Marine Zoology / Ichthyology, Senckenberganlage 25, D-60325 Frankfurt a.M., Germany ARTICLE INFO Keywords: Coral reef Resilience Ecosystem health Fisheries Red Sea Coral bleaching ABSTRACT Djiboutian coral reefs are poorly studied, but are of critical importance to tourism and artisanal fishing in this small developing nation. In 2014 and 2016 we carried out the most comprehensive survey of Djiboutian reefs to date, and present data on their ecology, health and estimate their vulnerability to future coral bleaching and anthropogenic impacts. Reef type varied from complex reef formations exposed to wind and waves along the Gulf of Aden, to narrow fringing reefs adjacent to the deep sheltered waters of the Gulf of Tadjoura. Evidence suggests that in the past 35 years the reefs have not previously experienced severe coral bleaching or significant human impacts, with many reefs having healthy and diverse coral and fish populations. Mean coral cover was high (52%) and fish assemblages were dominated by fishery target species and herbivores. However, rising sea surface temperatures (SSTs) and rapid recent coastal development activities in Djibouti are likely future threats to these relatively untouched reefs. 1. Introduction Coral reefs are crucial ecosystems both in terms of the biodiversity they possess and the ecosystem services they provide (Mace et al., 2012; Costanza et al., 2014). Globally reefs are being degraded by a combi- nation of climate change (Bellwood et al., 2004; Claar et al., 2018; Stuart-Smith et al., 2018), increased sedimentation from coastal de- velopment and land-use change (McClanahan and Obura, 1997; Erftemeijer et al., 2012), and ecological changes associated with the over-harvesting of reef resources (Hughes, 1994; Mumby et al., 2006). Based on our current understanding of reef resilience to coral bleaching and other human stressors, key indicators have been proposed to inform resilience-based management (Maynard et al., 2010; McClanahan et al., 2012). However, despite increased efforts in monitoring reefs in recent years (Arif et al., 2014, Hume et al., 2015, Osman et al., 2018, Hadaidi et al., 2018, Paparella et al., 2019), many areas of the Red Sea are lacking key information about the health and resilience of their reefs (PERSGA, 2010; Berumen et al., 2013), and therefore limiting appro- priate management strategies. Reef resilience to disturbance by coral bleaching is a function of factors that: (Muir et al., 2017) reduce a reef's exposure to bleaching by avoiding thermal stress; (2) reduce coral sensitivity to thermal stress; and (3) allow the coral community to recover if mortality occurs (West and Salm, 2003). These three components can be used to understand a reef's vulnerability to future climate pressures (Cinner et al., 2012; Belokurov et al., 2016) and be used to advise resilience based man- agement (McLeod et al., 2019). Bleaching exposure is reduced when the amount of light and temperature during a bleaching event (Coles and Jokiel, 1978). For example, high wave exposure is generally experi- enced on ocean facing fore-reefs, where wave energy and currents mix cooler deeper waters during stress events, whereas sheltered bays and inland seas suffer from greater surface warming and little mixing of the thermocline (West and Salm, 2003; Obura, 2005). Various mechanisms for coral community adaptation and coral colony acclimation to thermal stress have been proposed, including taxonomic shift to ther- mally tolerant coral genera (McClanahan et al., 2014), a shift to ther- mally tolerant zooxanthellae clades (Baker et al., 2004) and increases in photo-protective chemicals in coral tissues (Brown et al., 2002). Hence a reef's bleaching response during thermal stress is a combination of local physical conditions and the nature of the coral community. Re- covery from bleaching mortality is dependent on factors that promote coral regrowth, which include coral recruitment, suitable substrate, low pollution and sedimentation, low macroalgae cover and healthy her- bivorous fish populations (Fabricius, 2005; Mumby et al., 2006; Obura and Grimsditch, 2009). Management of local human stressors, such as overfishing and pollution, can enhance a reef's ability to recover and https://doi.org/10.1016/j.marpolbul.2019.07.040 Received 27 March 2019; Received in revised form 11 July 2019; Accepted 18 July 2019 ⁎ Corresponding author at: CORDIO East Africa, 9 Kibaki Flats, Kenyatta Beach, Bamburi Beach, P.O.BOX 10135, Mombasa 80101, Kenya. E-mail address: benjamindcowburn@gmail.com (B. Cowburn). Marine Pollution Bulletin 148 (2019) 182–193 Available online 17 August 2019 0025-326X/ © 2019 Elsevier Ltd. All rights reserved. T