By Mehryne Annooar, Varun Nathooram, Sarvesh Nikhilesh Gopal
The past few decades have been threatening to our corals due to numerous matters such as climate change, global warming and human activities. Due to the heat wave that occured in the late 1990s and early 2000s, the coral reefs of Mauritius experienced an intense process of bleaching which dropped the total percentage of healthy corals by 10 % in only two years. Now, what causes the bleaching? Coral bleaching occurs mainly when the temperature of the ocean floor increases drastically and during this process, the corals fail to adapt to the change in temperature in time. The exceptionally warm water leads to the death of corals and turns them white.
Why does coral bleaching occur?
Small organisms live on the corals, called zooxanthellae (a photosynthetic algae) have a mutualistic relationship with their host. The coral provides the algae with a protected environment and compounds they need for photosynthesis. In return, the algae produce oxygen and help the coral to remove wastes. Most importantly, zooxanthellae supply the coral with glucose, glycerol, and amino acids, which are the products of photosynthesis. The coral uses these products to make proteins, fats, and carbohydrates, and produce calcium carbonate.
The deadly 2000s
Between 2005 and 2007, a mild bleaching event occurred but the corals that had been affected by the heatwave of the early 2000s had not yet recovered. Around two years after the mild bleaching event, algal blooms in certain lagoon of the island resulted in the death of corals and fish.
The most important bleaching event that Mauritius experienced was in 2016 where 40% to 50% percent of bleaching was recorded around the island due to a heat wave. During the early months of 2016, the sea-water temperature increased above the usual average which is normally 29℃ causing a mild to severe bleaching according to the Mauritian Oceanography Institute (MOI). The sites that were the most affected by the bleaching event were namely Belle Mare, Flic en Flac and Ile aux Benitiers, where more than 65% of their live corals were partially bleached.
An example of coral bleaching (Source: Vox)
Coral bleaching is not the only culprit
Cyclones and rainfall
Between 2009 and 2015, the main cause of coral degradation were due to intense amount of rainfall which led to muddy waters to smother the corals. Flash floods that occurred in 2013 affected the northern regions of the island. In 2015, another flash flood affected the southern region of the island. The muddy water prevented the sunlight and enough oxygen to reach the corals which led to their destruction.
Overfishing and predators
Overfishing and bad fishing practices such as fisherman walking on corals cause the state of the corals to degrade and prevent their growth. Moreover, a specific species of starfish called the Crown of thorns feed on corals which slows down their growth.
A crown of thorn starfish feeding in corals (Source: Reef Magic Education)
Persistent increased in fresh water runoffs, reduced salinity, eutrophication (nutrient enrichment), increased sedimentation, erosion and pollution (marine pollution and pollution from inland activities), due to agricultural inputs, coastal development, deforestation, urbanisation and industrialisation. Ocean acidification due to global warming and coral diseases such as microbes including pathogens, white band diseases and Turpios sponge).
White band disease in coral (Source: marine savers.com)
Why are the corals important?
The Mauritian lagoons are home to more than 160 species of corals. It is a rather big amount if we compare it with other places which have only around 50 species of corals. The reefs exist in shallow waters covering 150 km around the coast of the island. According to Reefs Conservation Mauritius, an NGO that helps with the protection and restoration of corals, the growth rate of the coral colonies varies from 0.5 cm to 7 cm per year and the reefs of Mauritius are home to more than 4,000 species of fish. In addition to being the habitat of fish, corals also help control the level of carbon dioxide in the ocean by converting it into limestone shells. Moreover, the barrier reefs protect land and humans from natural threats such as tsunamis or sharks. The barrier reefs can significantly slow down the strong currents that are caused by tropical cyclones which would otherwise affect the coastline directly. Furthermore, the corals contribute largely to the Tourism Industry as many tourists visit the island in order to discover the beauty found under our waters.
Saving Corals Through Coral Farming
Coral farming is the process of taking fragments of corals and placing them underwater in “ coral nurseries” in order to allow it to grow into another coral. The process of coral farming is considered as impactful since from one big coral, we can grow many other coral. The process of coral farming is rather fast since as stated by the Professor Ranjeet Baghooli, we can grow 10 coral colonies from one coral colony that can grow in five years. Therefore, coral colonies can increase within a shorter amount of time it would have taken for the coral to grow in its natural habitat.
The process of coral farming is done in three different steps. The first step is to take fragments of healthy corals from bigger coral colonies. The second step is to place and attach the coral fragments in nurseries and wait until their growth. The final step is to place the fully matured coral from the colony into the ocean floor and allow them to grow.
A coral nursery (Source: eli-africa.org)
Coral farming is being done by the Mauritian Institute of Oceanography in Trou aux Biches as well as and NGO named Eco-SUD. The nursery of Eco-SUD is found in the South-East of the island in the surroundings of Ile aux Aigrettes.