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Double trouble for a coral reef

CNRS

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IMAGE: An illustration of the degradation of the coral reefs of the Samoan Islands (right), with a comparison with the preserved reefs of the Cook Islands (left) visited as part of... view more 

Credit: Gaëlle Quéré / Fondation Tara Expéditions

Organized over a period of almost three years, the Tara Pacific expedition has made it possible to study some of the most isolated coral reefs in the Pacific Ocean. Indeed, given their isolation, many of these reefs remain scantly documented. This is the case of the Samoan Islands in Polynesia. The scientists dropped anchor in November 2016 in Upolu, one of the islands of this independent state. Its geographic situation meant that there had initially been hopes of finding a well-preserved reef. The available data and satellite images also pointed to a rich diversity of coral ecosystems.

A very badly damaged reef

Yet, once there, the researchers found a coral reef in very poor health. That is why they decided to conduct a more extensive study of the reef than planned: instead of considering three sites, they selected 124 around Upolu--covering over 80 km of coastline--in order to inventorize their biodiversity. Each time, they noted their observations of the state of the corals and the behavior of fish. They also noticed that coral cover was below 1% in half of the sites they visited and below 10% in almost 80% of them. Another result, in most sites, was that the death of the corals was recent. According to the scientists, the coral cover might have been between 60% and 80% at some sites even just two years ago.

Stressed fish?

Moreover, by studying two species of fish they had encountered in previous places the schooner had moored (Moorea, Aitutaki, and Niue), the scientists observed that the Upolu fish were smaller and that the number of individuals in a shoal was on average four to eight times lower than in shoals around the three other islands. In addition, they observed a tendency for fish to flee, which is thought to reflect intensive fishing pressure.

The cause of this massive deterioration? On the one hand, global warming, which heightened a classic meteorological phenomenon (El Niño) in 2015-2016, entailing increased coral bleaching . On the other hand, local human activities, which are thought to exacerbate the effects of climate change.

Indeed, in those sites where there is significant anthropic pressure, 30% to 40% of the dead corals are already covered with macroalgae (this coverage ranges from between 0% to 3% in sites further away from any human presence). The release of chemical substances, sewage, and waste, as well as overfishing, could be having an impact on the capacity of already weakened or damaged corals to recuperate. In contrast, the scientists observed reefs in better health within marine protected areas, a sign of the effectiveness of certain forms of management.

These initial analyses will be followed by more in-depth investigations of the samples taken from Upolu. These samples will be fed into a database that, in time, will make it possible to compare the reefs and to distinguish and understand their capacities to resist environmental upheavals.

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Initiated by the Fondation Tara Expéditions and the Université Paris Sciences & Lettres, the Tara Pacific expedition (2016-2018) is supported by the CNRS, the CEA, the CSM, KAUST, and many other public and private partners including agnès b., Veolia, Fondation Albert 2, BillerudKorsnäs, and the Region of Brittany.

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