Saya de Malha Bank
Saya de Malha Bank is the largest submerged ocean bank in the world, located in the western Indian Ocean and spanning over 15,600 square miles (40,500 square kilometers). This vast underwater environment, part of the Mascarene Plateau, is characterized by its diverse ecosystems, including coral reefs, seagrass beds, and shallow banks, which support a rich array of marine life. The bank is crucial for fish species, acting as a significant feeding and breeding ground, particularly for threatened species like the green sea turtle and the blue whale.
The area remains largely unexplored due to its remoteness, which raises concerns regarding the sustainability of its fish stocks, especially with increasing fishing pressures from international fleets. Hand-line fishing, primarily conducted by Mauritius, has been a traditional practice since the 1950s, but recent industrial fishing activities have led to declines in local fish populations. Efforts are underway to implement better fishing practices and preserve this ecologically important region, which has prompted proposals for marine protection and conservation. Climate change also poses a threat to the unique ecosystems of Saya de Malha Bank, potentially disrupting the delicate balance of its marine life.
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Saya de Malha Bank
Category: Marine and Oceanic Biomes.
Geographic Location: Indian Ocean.
Summary: The largest submerged ocean banks in the world provide resources for fish and fishers.
Saya de Malha Bank is the largest submerged ocean bank in the world. Located in the western Indian Ocean, it covers an area of more than 15,600 square miles (40,500 square kilometers), representing one of the largest shallow tropical marine ecosystems on Earth. The bank is part of the underwater Mascarene Plateau, an underwater ridge east of Madagascar connecting the Seychelles and Mauritius, and is composed of the North Bank (or Ritchie Bank) and the much larger South Bank.
Because of its remoteness, the Saya de Malha Bank hosts some of the least explored tropical marine ecosystems. This highly productive oasis in the Indian Ocean sustains and represents an important food source for reef and pelagic fish. Diverse ecosystems such as coral reefs, seagrass beds, and shallow banks provide stepping-stone connectivity for species across the entire Indian Ocean. The great majority of the bank areas are located in international waters, and there is general concern regarding the sustainability of fish stocks targeted both by regional and international fisheries.
The banks were named around 500 years ago by Portuguese sailors who were exploring a new trade route to India. The origin of the name Saya de malha, ancient Portuguese words for military garb, apparently derives from the association of its impenetrable properties and the hazard that such mesh of shallow banks poses to navigation.
This shoal complex is situated in the great Mascarene Plateau, formed 20–40 million years ago by the volcanic hot spot of Réunion, which is still active today. These banks likely were islands that sank below the ocean surface as recently as 18,000 to 6,000 years ago. These days, the banks consist of a series of narrow underwater shoals with sandy bottoms, with depths varying from about 165 to 200 feet (50 to 60 meters), covered in major part by seagrass, surrounded by a shallower coralline rim that slopes to around 500 feet (150 meters) in depth. Completely detached from land boundaries, Saya de Malha Bank is surrounded by fairly sharp drop-offs up to 6,500 feet (2,000 meters) deep.
Biodiversity
The component banks of this biome are the tops of large seamounts, providing an extensive diversity of ecosystems throughout the depths and sustaining a great diversity of marine life. The plateau acts as a barrier to latitudinal water flows in the western Indian Ocean. The result is an upwelling system bringing deep, cold, and nutrient-rich waters to the surface, fueling primary productivity. In the deeper areas, impinging currents sustain food chains of small invertebrates and long-lived fish and coral species.
The banks represent one of the largest shallow tropical marine ecosystems, hosting the most extensive oceanic seagrass area in the world. Seagrass beds are among the most productive aquatic ecosystems; here, they cover 80 to 90 percent of the bottom—15,400 square miles (40,000 square kilometers). They provide a unique feeding area to one of the most numerous populations of the threatened green seaturtle (Chelonia mydas) in the western Indian Ocean. The remaining seafloor is covered by a diverse range of coral species and sandy areas.
The Saya de Malha Bank attracts significant numbers of tuna and billfish, and provides habitat to the world’s largest fish, the whale shark (Rhincodon typus). The area is also one of the rare breeding grounds for the blue whale (Balaenopterus musculus), the largest animal on Earth.
A limited number of oceanographic surveys have been conducted in the area, and many of the species and habitats of this remote region remain poorly documented. Great expeditions in the area range from the British Royal Navy surveys in 1838 to the Russian (1960s to 1980s) and Japanese fisheries expeditions, and the recent Agulhas and Somali Current Large Marine Ecosystem (ASCLME) surveys.
Human Impact
Most of the Saya de Malha Bank is in international waters, with Mauritius holding special fishing rights. Fishing is mostly by hand line, from dories deployed by mother ships based in Mauritius. International fleets operate deep-water fisheries, with purse-seiners and drifting long-liners targeting tuna species.
Saya de Malha provides the largest area for bank fishing on the Mauritius-Seychelles ridge. Most of the hand-line catch consists of Lethrinus mahsena, a reef fish locally known as dame berri. The fishing industry started in the 1950s, and by the early twenty-first century had an average catch of about 5,500 tons (5,000 metric tons) per year. In the mid-2010s, fishing ships began increasingly trawling the Saya de Malha Bank, leading to a decline in fish populations. The Mauritius government has been attempting to stop this overfishing and reduce the total allowable catches to sustainable levels.
Tuna fisheries have operated on an industrial scale since the 1950s in the southwestern Indian Ocean. Japanese, Taiwanese, and Korean fleets were later joined by fleets from Mauritius, France, and China. Estimates of Mauritius landings exceed 22,000 tons (20,000 metric tons) per year. The tuna fishery is an important industry worldwide, and the lack of information about it constitutes serious risk to the sustainability of open-water ecosystems. The Indian Ocean Tuna Commission is endeavoring to implement fishery observer programs.
In 2002, the region was proposed as an International Biosphere Reserve by the Lighthouse Foundation. In 2010, it was recommended as a High Seas Marine Protected Area by Greenpeace. The remoteness of the bank has protected the ecoregion from some threats (just as it has prevented much research and data collection). Prospecting for metals, oil, and gas beneath the seabed has returned weak results.
Climate change is also a major threat for the carbon-dominated food webs of the shallow banks. Accelerated absorption into sea water of carbon dioxide from the atmosphere can have deleterious effects, such as on the reproduction or shell formation in mollusks and crustaceans.
Bibliography
Ardron, Jeff, et al. “Defining Ecologically or Biologically Significant Areas in the Open Oceans and Deep Seas: Analysis, Tools, Resources and Illustrations.” International Union for Conservation of Nature. http://www.protectplanetocean.org/docs/GOBI‗Report‗2009.pdf.
“DSCC Submission on Bottom Trawl Fisheries on the Saya de Malha Bank .” Delegation of Deep Sea Conservation Coalition , 9 July 2021, www.apsoi.org/sites/default/files/documents/meetings/MoP-08-INFO-08%20Bottom%20Trawl%20Fisheries%20on%20the%20Saya%20de%20Malha%20Bank%20%28DSCC%29.pdf. Accessed 2 Sept. 2022.
Payet, R. “Research, Assessment and Management on the Mascarene Plateau: A Large Marine Ecosystem Perspective.” Philosophical Transactions of the Royal Society 363 (2005).
United Nations Environmental, Scientific and Cultural Organization (UNESCO): World Heritage Convention. “Saya de Malha Bank, Mascarene Plateau.” http://www.vliz.be/projects/marineworldheritage/sites/2‗Masc%20Plateau‗S%20Malha.php?item=The%20Indian%20Ocean.