Coral reefs of Mauritius

Reddish Acropora coral species with white buds on top growing in an aquarium

Mauritius island is almost completely enclosed by a 322 km coastline and a 150 km protective coral reef layer that shields a 243 km2 lagoon. Regions where reefs are absent include breaches 18 km wide in the south and 13 km wide in the west plus gaps near river mouths. It is believed that the formation of coral reef around the island started ⁓5000 years ago [1] and that the brain coral in the Blue Bay Marine Park in the south of Mauritius is the oldest known coral dating back some 1,000 years ago. Currently, more than 160 coral species have been identified in the waters of Mauritius [2].

Factors favouring coral growth

Mauritian waters provide an adequate environment for the growth of the beautiful reef around the island. These are:

  • The continental shelf is quite shallow and so allows just enough light to penetrate.
  • The water is clear and warm with an average temperature of ⁓25⁰ C.
  • The salinity of the sea lies around 35 ppt.
  • Spring tides have top peaks of 0.83 m that do not drown coral polyps.
  • The South East Trade Winds are moderate and do not infringe on coral development.

Types of coral reef in Mauritius

Five types of coral reef have been identified around Mauritius and its associated islands:

  1. Fringing reefs – Mauritius is mainly surrounded by fringing reef that protects the coastline leaving a shallow lagoon (15-45 m) in between.
  2. Atolls – Eventually, fringing reefs turn into atolls when volcanic islands get submerged by seawater; 22 atolls have formed in the St Brandon Archipelago of Mauritius.
  3. Barrier reefs – Barrier reefs are found far from the mainland, especially on the coast of Mahebourg where it extends from Ile de la Passe to ⁓9 km northwards [2]. Fringing reef is found in its inner layers for over a distance of 1.5 km laterally along the shore. A 2 km (width) by 20 km (depth) canal separates the inner reef from the rest of the mould.
  4. Patch reefs – As for patch reefs, they grow in shallow waters around the island where the seabed is high enough for coral to develop.
  5. Reef flats – Reef flats form as a result of fringing reefs that move seaward thus leaving thick reefs behind that eventually flatten due to water movement. Reef flats do not occur around mainland Mauritius but around Rodrigues.

The reef system

The reef front  

This is the backside of the reef that is in direct contact with the open ocean. As such, it is subject to pressure from the incoming waves and acts as a cushion against them. The reef front of Mauritius displays a rugged outline with pits and valleys in between. The foot of the reef may reach ⁓ -60 m. Different coral species grow at different depths of the sea:

  • 0 m – -5 m: branching and stony coral like Acropora, Pocillopora, Stylophora, Millepora
  • -5 m – -15 m: massive corals
  • -15m – -20 m: massive corals like Goniophora, Favia and Porites
  • -25 m – -30 m: skeletal corals
  • below -30 m: most types of corals such as Antipatharians and Alcyonacea grow together with algae.

The reef platform

This is the upper surface area of the coral reef that can be seen from above. Mauritian reef exhibits a closed network formation reaching 20 m on average, crisscrossed by rills that allow the free passage of water from the open ocean into the lagoon. The calm side of the reef is covered by stony corals, sea urchins and other echinoderms while the windward segment is dominated by skeletal coral and sea anemones. 

The reef flat

It is the part of the reef that faces the coast. The width of the reef varies from place to place but in general, can be as big as hundreds of metres and are larger on the east coast. The depth of the reef is less than 1 m during spring tide but can go as deep as 20 m where canals are present. Only isolated colonies of Acropora develop due to siltation from the land.

Canals and basins

The lagoon of Mauritius is characterised by the presence of canals and basins along the seafloor such as at Blue Bay. They are often as wide as 0.5 m to 1.5 km and as deep as 6 m to 35 m. Polyps grow on the sides of the reef while the middle part is covered with sediment and hinterland materials that prevent polyp growth.

Importance of coral reefs in Mauritius

Environmental

As a small island, the coral reefs around Mauritius provide very important environmental and socio-economic benefits. The reefs provide shelter and food to several beautiful creatures such as fishes, clams, turtles and seahorses that also add to the marine biodiversity of Mauritius. In addition to this, they act as spawning grounds for many marine organisms and are vital in nutrient recycling in the ocean. In the case of Mauritius especially, the massive reef that surrounds the island plays a key role in protecting the coast against violent waves, tsunamis and storm surge.

Scissor tail sergeant fish in front of purple coral and above yellow coral is a commonly found fish in the coral reefs of Mauritius
The beautiful Scissor tail sergeant fish can be commonly seen on the coral reefs of Mauritius

Socio-economic

As a matter of fact, the reef system that surrounds the island is the reason for the calm lagoon and clear water. Basically, the tourism industry of Mauritius is based on this type of beach travel and marine activities are also one of the most common recreational activities of the local population. With a shift in the tourism sector to sustainable practices and eco-activities, holiday planners and tour agencies are exploiting the marine heritage to give tourists unforgettable underwater experiences. Thus tourists can enjoy the magnificent coral ecosystem through diving, undersea walks and submarine trips. Similarly, commercial fishing for local consumption and exports, as well as recreational fishing by locals and tourists, depend on healthy coral reefs.

Research

Coral reefs are known to be sources of food and drugs: future research into the coral reefs of Mauritius may provide such invaluable data and resources. In addition to that, fundamental information is locked up in reefs that can help to get insight into past events and predict future impacts. Past sea level rise estimation in Mauritius has been carried out by analysing the coral reef at Pointe aux Sables in this way [3].

Threats to the coral reefs of Mauritius

Anthropogenic threats

In the past decades, some 50-60% [1] of live coral cover has been lost due to a number of anthropogenic and natural causes. Human activities such as dredging and coastal development have increased siltation into the lagoon causing many coral polyps to die. Similarly, mangrove trees that trap sediment and pollutants have been cleared in many regions thus allowing sediment and pollutants to reach and kill corals. Inland and offshore pollution such as agricultural runoff, untreated waste, recreational activities and discharges from boats directly affect corals. Processes like eutrophication, decreased salinity and light penetration and changes in sea chemistry further prevent coral reefs from growing.

Natural causes

One of the key reasons for coral death in Mauritius is attributed to ocean warming and subsequent coral bleaching. These bleaching events are normally associated with an increase in sea temperature due to increased solar radiation (global warming) or to El Nino events. For instance, the most severe bleaching episode in 1998 caused the loss of more than 10% of the live coral cover while El Nino event in 2016 also had a great impact on the coral reefs. Cyclones too damage much of the coral reefs as has been noted after the passage of cyclone Davina, Dina and Kalunde. Violent waves induced by cyclones damage the reefs directly, intense rainfall on the coast leads to suspension of sediment into the sea and there can even be changes in the coastal landscape thereby altering ocean currents. In the same manner, flash floods have been observed to negatively impact on coral growth as they lead to increased turbidity in the ocean. Predators like the crown of thorn starfish prey on corals preventing their regeneration or inhibiting the development of the slower-growing corals. Several coral diseases have also been recorded to kill many corals.

Coral reef management in Mauritius

Given the island’s geographic position and economic dependence on its coral resources, the government of Mauritius has come up with a number of laws and policies regarding the management and protection of its coral reefs. Fishing reserves, marine parks and delimitation of no-take zones have been proclaimed in addition to seasonal fishing with the appropriate equipment. There has also been the prohibition of sand and coral mining, shell collecting and a moratorium on sea cucumber exploitation. Several legislations and guidelines are in place concerning coastal development such as Environmental Impact Assessments, zonation, setback rules, maritime contingency plans as well as the national Integrated Coastal Zone Management framework. New techniques are being applied to restore degraded reefs including coral farming and coral propagation to different sites using fertilized eggs during spawning events.

References:

  1. Moothien Pillay, R., Terashima, H., Venkatasami, A. and Uchida, H. (2002). Field Guide to Corals of Mauritius. Albion, Mauritius: Albion Fisheries Research Centre
  2. Saddul, P. (2002). Mauritius, A Geomorphological Analysis. Moka, Mauritius: MGI, pp 262-267
  3. Montaggioni, L.F. and Faure, G. (1997). Response of reef coral communities to sea level rise: a Holocene model from Mauritius (Western Indian Ocean). Sedimentology, 44, pp. 1053-1070.

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