Coral Bleaching Disturbs Structure Of Fish Communities

Research work reported on recently by an international research team*, including an IRD scientist, brought out evidence of the impact on the fish communities of a mass bleaching event resulting from the 1997-1998 El Niño climatic episode. The investigation was wide in scope, focusing on more than 60 coral reef sites in the Indian Ocean, including nine located in Marine Protected Areas.

Changes in diversity, size and composition of fish communities were found to follow the decline in the coral reef’s health. Nevertheless, the status of no-take Marine Protected Areas, where fishing is strictly forbidden, appeared to have little impact on the recolonization of such waters by corals. The scientists therefore recommend the setting-up of reserves specially devoted to coral reef conservation.

Reef-building coral may be defined as the result of a symbiotic relationship between microscopic dinoflagellate algae, the zooxanthellae, and an animal organism, the polyp. In terms of biodiversity, coral reefs are often compared with the tropical rain forests. The reef ecosystem harbours thousands of species, whose complex interactions are still largely unfathomed. The main destructive factor facing rain forests is deforestation. The equivalent threat to coral ecosystems is bleaching. The sea surface temperature needs to rise only by a small number of degrees for the polyp to expel the zooxanthellae, the very element that is vital for its survival. Without these microscopic algae, the coral loses its colour, no longer receives the nutrients essential for its development and hence eventually dies.

Scientists have been working on the coral bleaching process for many years, but few studies have made it possible to assess the large-scale impact on reef fish communities. Recent work by an international team in the Indian Ocean, including an IRD scientist, has provided keys to improved measurement of the consequences of a coral bleaching event, linked to a high-intensity El Niño event in 1997-1998, on fish communities. February 1998 saw temperature rises resulting from this regional climatic anomaly provoke mass bleaching - involving almost half the Indian Ocean’s corals. That represented the most severe event of this kind since biologists began to study this ecosystem.

Data concerning the size, structure, diversity and trophic composition of fish communities of 66 coral sites of seven different countries (Maldives, Chagos Archipelago, Kenya, Seychelles, Tanzania, Mauritius and Réunion Island), collected in the mid 1990s, were compared with more recent ones gathered in 2005. A regional-scale picture of the 1998 bleaching event’s impact on the coral reefs was produced by superimposing the two sets of information. Analysis showed that the decrease in the proportion of living coral and modification of its architectural complexity were two decisive factors behind changes to the structure of fish communities in the reef ecosystem.

After the corals die, turf algae and macroalgae rapidly invaded the space left free by the dead corals, leading to increased uniformity of the habitat. The mortality resulting from the bleaching particularly affects the reef-building corals. The calcareous constructions the latter build up serve as shelter and nursery for dozens of fish species. The huge decline of corals therefore affects those fish that depend closely on coral colonies, either as feeding grounds, like the polyp-eating butterflyfish, or to find protection, like the small-sized damselfish. When their numbers recede, these species are often replaced by herbivore fish (parrotfish, surgeonfish and so on) adapted to consuming certain algae proliferating on dead coral. Some, such as damselfish of the Stegastes genus, can then invade the coral reefs smothered in algae, as happened on several sites following the bleaching of 1998.

Moreover, nine out of the 60 or so sites studied are located within Marine Protected Areas where fishing has been strictly forbidden since the mid 1960s. As can be expected, the scientists have observed, after bleaching, a higher density of fish and a larger size among specimens in these marine reserves. The corals, however, did not manage to recolonize these protected zones any more rapidly than in ordinary areas. The weak level of coral regeneration appears partly linked to the fact that these strictly demarcated protection areas are close to the Equator, where the ocean warming of 1998 was most intense.

These results highlight its deleterious effects on the reefs, show that it is necessary to designate marine protected areas specifically dedicated to coral conservation. The creation of a network of marine conservation zones far enough away from the Equator to limit as much as possible the global-warming related rise in sea surface temperature, could at regional level, represent refugia effective for conservation both of the corals and of the fish species closely bound to the reef ecosystem.

*This research project was run jointly with scientists from Newcastle University (United Kingdom), the Wildlife Conservation Society, New-York (USA), James Cook University, Townsville (Australia), Université de la Réunion, Université de Marseille and Université de Perpignan (France).