Southern Madagascar: the ‘deep south’

Location/ Description/ Jurisdiction/ Features of Potential Outstanding Universal Values/ Threats/ Management status/ Geographic scale and site integrity/ Other sites in the region / Key References

Locator map
© David Obura

Complex currents affected by the submarine plateau result in eddies and upwelling. To the west, the shallow currents feed into the Mozambique channel and to the Agulhas Current in South Africa.
© diMarco et al. 2000

Location - The zone of interest is from Lokaro/Ste Luce (old name, Fort Dauphin, in the east) to Androaka (in the west). The continental shelf is very narrow between the Linta and Mangoky deltas and falls steeply down the continental slope.

Description - The shallow marine habitats of the coast are mainly rocky and experience very rough conditions exposed to the south. There is minor coral reef development at the east (Lokaro, Ste Luce) and the west (Androka, banc de l’Etoile) extremities. Coastal habitats are varied, with distinctive coastal dune formations, such as at the Mangoky delta. The coastal dune systems were formed during the Quarternary, with a width of up to 30 km. in places, the dunes are carved into cliffs up to 150 m high.
Extending south from Madagascar, an extensive underwater plateau or ridge varies from about 1000 to 2500 m deep, for a distance of nearly 1000 km. At its southern end it forms a shallow platform that reaches to 100 m below the surface. The platform was formed by basaltic extrusion from the Marion hotspot during the Cretaceous, as Antarctica and Madagascar moved apart. The region experiences complex oceanography caused by the strong boundary current of the East Madagascar Current impinging on the undersea Plateau, resulting in strong coastal and offshore upwelling, eddies and turbulence. These cause a large phytoplankton bloom in the austral summer that fertilizes waters downstream and into the southern Mozambique Channel. Eddies generated over the plateau may progress into the Mozambique channel, interacting with those of the channel and potentially move north up the west coast of Madagascar.
The plateau extends southwards into temperature waters and ecoregions, resulting in mixed tropical and subtropical species and habitats. Even on land and in shallow waters, this region is poorly documented. A recent expedition ‘Atimo Vatae’, focused on the algae and invertebrates of the shallow and upper-slope marine fauna, has already shown preliminary results reporting kelp beds and over 500 species of algae, richer than the tropical algal flora of Mozambique. By contrast, diversity of animal species is lower than in tropical areas, but with extremely high levels of endemism and shared species with subtropical South Africa. Ascidians are among the first groups analysed, revealing 20% new species, 26% shared with S. Africa, and 31% shared with tropical areas. Other groups with preliminary numbers include molluscs (1200-1500 species), decapod crustaceans (766 species) and fish (253 species).
Offshore, the undersea Plateau, because of its upwelling and productivity, provides critical feeding grounds for multiple marine species, including seabirds, large fish and marine mammals. The red-tailed tropicbird and Barau’s petrel (endemic to and nesting on Reunion) feed on the plateau, indicative of a high number of species that also do this. Blue, sperm and humpack whales aggregate in these waters due to its raised productivity, with estimated population sizes of about 450 individuals for pygmy blue whales. Humpback whales also use the SW and SE coasts (near Toliara and Lokaro, respectively) as breeding grounds, and for nursing by mothers and calves.

Jurisdiction - Madagascar

Features of Potential Outstanding Universal Values

Criterion viii - Geology and oceanography
Geology: The Madagascar Plateau is a product of Cretaceous hotspot activity as Madagascar moved north away from it.
Oceanography: Interaction of the plateau with the strong western boundary current, the East Madagascar Current results in high turbulence, upwelling and productivity.

Criterion ix - Ecology, species and evolution
Because of its southerly location, this is a transition zone (ecotone) in the Indian Ocean between the tropical waters and the temperate waters, at the crossroads of the fauna of South Africa and that of the Indo-Pacific, with an African affinity as one approaches the Mozambique Channel.
Very specific communities adapted to local conditions - high energy, upwellings, cooler waters.
High levels of endemism have been found, 25% for mollusks, with many new species likely to be described in coming years.
Leatherback, loggerhead and green turtles are found in the region, one of the few locations important for leatherbacks in the WIO.

Criterion x - Habitats and conservation
Large coastal dunes, lagoons and coastal ponds, forming unique coastal habitats and wetlands.
Shallow benthic communities dominated by hard substrate communities, with small isolated coral reefs at the extremities.
The highly productive waters of Madagascar’s ‘Deep South’ are critical feeding grounds for the highly migratory species of the region, including seabirds and cetaceans.

Typical landscapes of the deep south include sandy bays (top) backed by large dunes and rocky shores pounded by ocean swell from the Southern Ocean (bottem).
© Bruno de Reviers / MNHN-PNI

Threats – The area is highly remote, with little development on land. Thus offshore fisheries by commercial fleets, targeting fish that feed on the high productivity off the plateau will likely develop as other fishing grounds become depleted.

Management status - There is currently no management structure for the overall area, though one coastal site, Faux Cap is a Ramsar site.

Geographic scale and integrity issues– This is a large and remote region, with very rough and inaccessible coastlines. It will provide significant challenges to management, though at the same time, these help protect it from impacts, and confer its values.
The Madagascar plateau extends southwards for 1,000 km into the High Seas. For initial consideration, limiting a site to the EEZ, 200 nm from the coastline, to encompass the main nearshore components of the system, is advisable. The High Seas component of the plateau may be suitable for consideration under the EBSA process (Ecologically and Biologially Sensitive Areas). Two hundred km north of the point where the western edge of Madagascar Plateau, a submarine canyon leading into the Bay of St. Augustin (at Toliara) results in high diversity of cetaceans due to accessibility of deep water, offshore species. The canyon is also an important site for coelacanths. These may justify extending the boundary of this site to include the Bay of St. Augustine.

Site type – May be considered as a single entire site, or potentially a serial site of the key locations for each of the primary attributes of OUV.

Other sites in the region – This is a unique site in the Western Indian Ocean.

Endemic species of Madagascar’s ‘deep south’ include the snail Lyria patbaili (left) and a likely new species of crab in the family Hymenosomatidae (right).
© Bob Abela / MNHN-PNI (left), © TIN-Yam Chan / MNHN-PNI (right)

Key References – Battistini (1996); Best et al. (2003); Bouchet (2012); de Ruijter et al. (2004); Di Marco et al. (2000). --> References