Coral Bleaching Disturbs Structure Of Fish Communities

Editor's Note

If there are still any doubters, more and more research is coming out on the effect of global worming on the bleaching on the coral reefs.  

There is no longer any shadow of a doubt about the impact of global warming on coral reefs. A rise of a few degrees in sea surface temperature induces the expulsion of essential microscopic algae which live in symbiosis with the coral. This process is the cause of coral bleaching and is well known to scientists, but few large-scale studies have dealt with its effects on the structure of communities of hundreds of species of reef-colonizing fish.

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.