Phase II – The effects of Coral Bleaching on Coral Reef Fish

Increase in sea surface water temperature can cause coral bleaching – a condition whereby corals lose their symbiotic algae (zooxanthellae) causing the coral to lose its colour and turn white. Mass coral bleaching events occurred in all coral reef regions of the world in 1998, resulting in 16% mortality of the world´s reefs. The 1998 El Nino event caused the greatest disturbance to date in the Indian Ocean where large areas experienced 50 to 99% mortality of living corals. This project aims to elucidate what happened to coral reef fish assemblages in response to the coral bleaching in a medium to long-term period (7-8 years) and derived implications to fisheries, tourism, and biodiversity conservation. This project was implemented in seven countries in the WIO namely, Kenya, Tanzania, Madagascar, Seychelles, Mauritius, Maldives, and Reunion. The targeted beneficiaries of this project included local communities, scientific community, and decision and policy makers.

To achieve the objectives several methods were deployed: (i) Meta-analysis of historical data to determine the changes and the effects of coral mortality, management, and remoteness on fish communities, (ii) pre- and post-bleaching fish surveys to establish likely responses to disturbance on coral reef resources, (iii) long-term fishery dependent surveys to determine their dependence on coral reef resources, (iv) modeling of fishery effects to predict the observed changes and to develop scenarios for the effects of climate change on reef ecology and fisheries under different management objectives, and (v) socioeconomic studies through household surveys and key informant interviews.

The study found high spatial variability in the extent and effect of the 1998 coral bleaching. Coral cover declined by ~40% across the 1998 bleaching event, with the highest mortality in the central and northern regions of WIO. The most severely affected reef areas were southern India, Sri Lanka, central atolls of the Maldives and Granite Seychelles, Northern Arabian/Persian Gulf, Gulf of Oman, Chagos, Kenya, southern Tanzania and southern Seychelles. Also noted were reductions in obligate corallivores proportional to the loss of coral. When species were grouped by their maximum attainable size, a clear trend was apparent for species <20 cm, but no relationship was observed for large-bodied (>20 cm) fishes, which are the main target of fisheries, and this eventually made the detection of the impact of coral bleaching on the fishery difficult. When different sites were ranked according to their adaptive capacity, it was found that sites in Seychelles had both high adaptive capacity and high environmental susceptibility. High adaptive capacity in the Seychelles can be used to develop alternatives to coral reef resource dependence and efforts to reduce the effects of climate change. On the other hand Mauritius had high adaptive capacity, but low environmental susceptibility. The high adaptive capacity in Mauritius can increase the chances for successful self-initiated recovery and protective management. Sites in Madagascar, Tanzania, and Kenya all showed low to moderate adaptive capacity, but highly variable environmental susceptibility.

A key finding and recommendation of this research at a regional scale is the importance of large-scale biogeographic planning as a means to prepare for climate change and aligning the environmental susceptibility of the reefs with the appropriate management. This project has identified areas that will and will not suffer the most change from climate change and this needs to be a basis for regional planning and management priorities. Incorporating adaptive capacity and environmental susceptibility into conservation planning will represent a significant shift in approaches to resource conservation. Simulation models suggest that protecting coral reef herbivores can be achieved by restricting the use of spear guns and beach seines, therefore, efforts to reduce their use will improve the chances for their reduced effects and recovery.

Future studies could consider extending the framework developed from this research by considering additional axes such as local impacts on ecosystems, the strength of governance systems, or the ability of ecosystems to provide goods and services.