Marine Ecosystems
Sea Turtles
Seychelles hosts four of these species including the 'Critically Endangered’ Hawksbill (Eretmochelys imbricata) and 'Endangered' Green turtle (Chelonia mydas), both of which nest and forage around the outer islands, and the 'Endangered' Leatherback (Demochelys coriacea) and Loggerhead (Caretta caretta) turtles. The Hawksbills nest on more than 20 islands, whereas the Green turtles are more abundant in the southern Aldabra group.
Coral Reefs
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Coral reefs are found throughout the western Indian Ocean and the Seychelles. The reefs of the Seychelles can be classified into three major groups: fringing reefs, platform reefs and atolls. The reefs around the outer islands are mainly either platform reefs or atoll reefs. Within each of these types, raised reefs are also found. The reefs of the granitics have received more research attention over the years than those around the outer islands, although there have been more recent efforts to address this imbalance.
The coral reefs around the atolls at depths >38m typically support Gorgonian sea fans, and the ahermatypic corals such as Dendrophyllia spp. and Tubastrea spp. on a sand and gravel plan. Other species of unattached scleractinian colonies (39 species, belonging to 22 genera |
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and 9 families), which are usually fixed to the reef slope substratum, have been found at 38m. Between 38m to 28m, encrusting corals are more dominant (e.g. Pachyseris sp.). Between 28m to 14m, the coral communities are typically dominated by faviids and mussids (e.g. Echinophyllia spp., Pachyseris spp., Symphyllia spp., Lobophyllia spp., and Goniastrea spp.). Shallow reef slopes from 14m to 6m are often dominated by a mixture of hard and soft corals, with an increasing abundance of the branching acroporids. Some corals such as the blue coral Heliopora coerulea, may form vast monotypic beds (30–40% of the substratum cover is common around some islands). In waters <6m the reefs maybe dominated by Acropora and other hermatypic species.
There are an estimated 320 species of scleractinian corals found around the Seychelles, all of which are listed on the IUCN Red List, and some of which are listed as 'Endangered', 'Vulnerable' or 'Near-threatened'. Surveys in 1992 to 1993 recorded 51 genera of corals, and 161 species. A more recent survey found 139 species of scleractinian corals around Farquhar and 190 species in total around the Amirantes.
There was an extensive mass coral bleaching event in 1997/1998, as a result of anomalously high sea water temperatures and calm conditions, which impacted the entire western Indian Ocean and resulted in the mortality of corals around both the inner and outer islands of the Seychelles. The impact of this event was variable: corals in the lagoons or on reefs that were already acclimated to warm temperatures, or were exposed to localized upwelling or strong currents, demonstrated greater resilience and resistance. Recovery since this event has been equally variable and been influenced by cyclones and other threats.
There are an estimated 320 species of scleractinian corals found around the Seychelles, all of which are listed on the IUCN Red List, and some of which are listed as 'Endangered', 'Vulnerable' or 'Near-threatened'. Surveys in 1992 to 1993 recorded 51 genera of corals, and 161 species. A more recent survey found 139 species of scleractinian corals around Farquhar and 190 species in total around the Amirantes.
There was an extensive mass coral bleaching event in 1997/1998, as a result of anomalously high sea water temperatures and calm conditions, which impacted the entire western Indian Ocean and resulted in the mortality of corals around both the inner and outer islands of the Seychelles. The impact of this event was variable: corals in the lagoons or on reefs that were already acclimated to warm temperatures, or were exposed to localized upwelling or strong currents, demonstrated greater resilience and resistance. Recovery since this event has been equally variable and been influenced by cyclones and other threats.
Fishes
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There are more than 1,000 coastal fishes that have been identified in Seychelles, which includes 400 reef-associated species and 2 endemic species, which includes the Seychelles clown fish (Amphiprion fuscocaudatus). There are two new species of sharks (Squalus lalannei and Centrophorus seychellorum) that have recently been described. The whale shark (Rhincodon typus) is a regular migrant to Seychelles waters. The outer islands also support important spawning aggregations of Grouper, some of which are included on the IUCN Red List (e.g. the "Endangered" Napoleon wrasse Cheilinus undulatus, "Vulnerable" Black-saddled Coral Grouper, Plectropomus laevis, and the "Near-threatened" Brown-marbled Grouper Epinephelus fuscoguttatus).
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The 'Endangered' Napoleon wrasse (Cheilinus undulatus), the largest of the Labridae was observed at nearly all (86%) sites during recent surveys around Farquhar, ranging in size from 0.5m (juveniles) to 2m (adult males). The abundance and healthy size structure of this species at Farquhar, rivals that of Coco Keeling, and suggest an important need for conservation. Likewise, the Bumphead parrotfish (Bolbometopon muricatum), the largest of the parrotfishes (~1.5m in length), which is also listed as 'Threatened' on the IUCN Red List, was observed at 36% of the sampling locations with one school in excess of 500 individuals. The abundance of these fish populations is in sharp contrast to other parts of the world where many of these aggregation sites have collapsed.
There are 3 regionally endemic species of shark, the Bluespotted bambooshark (Chiloscyllium caerulopunctatum), Harlequin catshark (Ctenacis fehlmanni) and Short-tail nurse shark (Ginglymostoma brevicaudatum). The presence of these species in Seychelles waters is unknown. Both the Giant Manta ray (Manta birostris) and/or Reef Manta (Manta alfredii), which are both listed as 'Vulnerable' on the IUCN Red List, are thought to occur around the islands in the Indian Ocean. Large aggregations of Manta are known to occur around the St Josephs and Alphonse Group. The shallow lagoons of several islands also serve as a breeding ground, nursery and feeding ground for several elasmobranch species such as: sicklefin lemon shark (Negraprion acutidens), tawny nurse shark (Nebrius ferrugineus), blacktip reef shark (Carcharinus melanopterus) and sting rays (Pastinachus sephen, Himantura granulate and Urogymnus asperrimus). Other species known to occur in the southern Seychelles and outer islands region include the 'Endangered' greater hammerhead sharks (Sphyrna mokarran), giant guitarfish (Rhynchobatus djiddensis), 'Vulnerable' Oceanic whitetip (Carcharhinus longimanus) and Sandbar shark (Carcharhinus plumbeus).
There are 3 regionally endemic species of shark, the Bluespotted bambooshark (Chiloscyllium caerulopunctatum), Harlequin catshark (Ctenacis fehlmanni) and Short-tail nurse shark (Ginglymostoma brevicaudatum). The presence of these species in Seychelles waters is unknown. Both the Giant Manta ray (Manta birostris) and/or Reef Manta (Manta alfredii), which are both listed as 'Vulnerable' on the IUCN Red List, are thought to occur around the islands in the Indian Ocean. Large aggregations of Manta are known to occur around the St Josephs and Alphonse Group. The shallow lagoons of several islands also serve as a breeding ground, nursery and feeding ground for several elasmobranch species such as: sicklefin lemon shark (Negraprion acutidens), tawny nurse shark (Nebrius ferrugineus), blacktip reef shark (Carcharinus melanopterus) and sting rays (Pastinachus sephen, Himantura granulate and Urogymnus asperrimus). Other species known to occur in the southern Seychelles and outer islands region include the 'Endangered' greater hammerhead sharks (Sphyrna mokarran), giant guitarfish (Rhynchobatus djiddensis), 'Vulnerable' Oceanic whitetip (Carcharhinus longimanus) and Sandbar shark (Carcharhinus plumbeus).
Marine Mammals
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The Indian Ocean has been a whale sanctuary since 1979. According to the species list prepared by MCSS, there are 25 species of cetaceans known to frequent the waters around the Seychelles; these include 'Endangered' whale species, such as the Sie Whale (Balaenoptera borealis), Blue Whale (Balaenoptera musculus) and Fin Whale (Balaenoptera physalus); and 'Vulnerable' species such as the Sperm Whale (Physeter macrocephalus), and 8 species of dolphin. Dolphins as well as Humpback whales and Sperm whales have all been observed around the majority of the Outer islands. In 2009, ICS staff Danielle Jupiter and Lisa Thompson found a new species of whale washed up on Desroches Island. The species was officially recognized as Mesoplodon hotaula or ‘Deraniyagala's Whale’. The results were presented to the International Whaling Commission in June 2012.
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While marine mammal mortality resulting from fisheries interactions are considered to be lower in the WIO than in other regions of the world, there are two coastal species that are particularly affected by human activities: the Dugong (Dugong dugon, classified as ‘Vulnerable’ by IUCN); and the Indo-Pacific Bottlenose Dolphin (Tursiops aduncus, classified as ‘Data Deficient’ by IUCN. Dugong are the most endangered and threatened marine mammal in the WIO. There is only a small population of dugong found around Aldabra and the only known viable population is located in the Bazaruto Archipelago, in Mozambique.
Marine invertebrates
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The coral reefs and other benthic habitats around the Seychelles also host a wealth of invertebrates, which have been poorly studied to date. There are still unidentified species even from among the commercially important invertebrate groups. The molluscan fauna was researched around Cosmoledo and Desroches between 1987 and 1989 in an attempt to identify species that could be exploited by the artisanal fishery. These surveys revealed 450 species of bivalves between 0 to 20 m depth, including 5 exploitable species (three species of Trochidae and two species of Turbinidae) and a further 63 species of potential commercial interest. There are over 150 echinoderms including 33 species of sea urchin, 32 starfish, 35 sea cucumbers (holothurians), 9 crinoids, and 44 ophiuroids. There are a large number of sponges that are known to occur in the Seychelles,
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18% of which are thought to be regional endemics. 165 shrimp species have been identified in Seychelles, of which many are endemic. In the Seychelles, one of the most important commercial invertebrate fisheries is the sea cucumber fishery. Sea cucumbers are harvested and mainly processed as bêche-de-mers for export. There are more than 20 commercially viable holothurian species, but there are six species which dominate catches. The most commercially important sea cucumber is known as "Pentard" or teatfish, which is a species of Holothuria sp. that is new to science and as yet to be named, then (in descending order), white teatfish (H. fuscogilva), prickly redfish (Theleonata ananas) and black teatfish (H. nobilis), Sandfish (H. scabra) and blackfish (H. edulis). The first four species have dominated reported catches from 2000 to 2012, accounting for 83% of the total catch. The highest catches include Owen Bank, the Granitic Islands, the Amirantes and Coetivy. The only places where “Pentard”’ are not dominant include Ile Plate and the north of the Amirantes.
Sea Grasses
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Seagrasses are known to provide a food source for many organisms, including the 'Endangered' Green sea turtle (Chelonia mydas) and Dugong (Dugong dugong). Many other marine species also depend on seagrass habitats as feeding and nursery areas. Seagrasses help stabilise sediments and maintain water quality. The blades slow the flow of water and encourage the deposition of fine sediments, helping to keep the water clear, while the root systems bind and stabilize sediment particles. More recent research has also highlighted the role that seagrasses play in terms of storing carbon and moderating water chemistry. Research has now demonstrated that seagrasses can reduce the acidity of seawater, by increasing seawater pH by up to 0.38 units (and aragonite saturation state). Although the shift in pH values may vary depending on water residency time, the presence of seagrasses could significantly benefit calcifying organisms including scleractinian corals downstream of seagrass beds.
Seagrass beds are found throughout the western Indian Ocean. The Seychelles seagrass beds may be composed of 13 different species (out of 50 known globally). Extensive seagrass beds are known to occur around the outer islands, although the exact extent and total number of seagrass species is currently unknown. Eight of the species known to occur around the Seychelles are included on the IUCN Red List as 'Least Concern' including: Enhalus acoroides, Halophila decipiens, Halophila ovalis, Thalassia hemprichii, Cymodocea rotundata, Cymodocea serrulata, Halodule uninervis, Syringodium isoetifolium, and Thalassodendron ciliatum. |
Mangroves
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Mangroves are found throughout the Indian Ocean, and provide a critical nursery habitat for juvenile species and help to protect shorelines from storms, winds, waves, and floods. Mangroves can also prevent erosion, as their root systems stabilize sediments, and they can help to maintain water quality and clarity, by filtering nutrients and trapping sediments originating from land. More recent research has also highlighted the role that mangroves play in terms of storing carbon.
Around the outer islands of the Seychelles mangroves are particularly well developed in the lagoons of Aldabra, Cosmoledo and Astove. There are also important areas of mangrove around St Joseph’s Atoll, Farquhar (South Islands), Poivre (South Island), and Alphonse Group (St François atoll). The Seychelles host 9 species of mangrove, which is the highest number of species in the western Indian Ocean region (along with Madagascar and Zanzibar). Eight of these species are included on the IUCN Red List, as 'Least Concern' including: Avicennia marina, Lumnitzera racemosa, Pemphis acidula, Sonneratia alba, Bruguiera gymnorhiza, Ceriops tagal, Rhizophora mucronata, Xylocarpus granatum. The most abundant species found around the outer islands tends to be the red mangrove (Rhizophora mucronata). |
Seabirds
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There are over 30 species of seabird known to occur in the western Indian Ocean region, and the islands are generally considered important areas for nesting, resting, breeding and feeding for these seabirds. The Seychelles hosts vast numbers of breeding seabirds, both in the granitic and outer islands. Some colonies host more than one million birds and are among the largest in the Indian Ocean and the world. Although Seychelles is not situated along any important migratory route, many migratory species occur regularly.
Desroches and Alphonse only support small numbers of seabirds due to the level of historical disturbance, although both islands do support colonies of Wedge-tailed Shearwaters and other migratory wading birds and terns. |
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Farquhar is however recognised as an Important Bird Area (IBA), by Birdlife International due to significant numbers of breeding seabirds found on the islets. Goëlettes island for example supports a huge breeding colony of Sooty Terns (Onychoprion fuscatus) (200,000 to 400,000 pairs), Roseate Terns (Sterna dougalii) (15–20 pairs), and Black-naped Terns (Sterna Sumatrana) (40 pairs). Other significant breeding seabird species include Red-footed Booby (Sula sula) (50–70 pairs), and Brown Noddy (Anous stolidus) (10,000 pairs). Banc du Sable also hosts a previously unknown colony of Black-naped Terns (10-20 breeding pairs). This colony and the one on Goëlettes makes Farquhar one of only 6 known breeding sites for Black-naped Terns in Seychelles, which hosts the entire known population of the African region.
Both Bijoutier and St François meet IBA criteria for four species of congregatory water-birds: Black-naped Tern, Saunders’ Tern (Sterna saundersi), Crab Plover (Dromas ardeola) and Ruddy Turnstone (Arenaria interpres). D'Arros is an IBA due to an introduced population of the Seychelles Fody. Other land birds also include the Madagascar turtle dove. There is a strong case to extend the IBA to St Joseph, which has the largest population of Roseate terns (about 300 pairs) and Wedge-tailed shearwaters (about 300,000 pairs) in the outer islands. Black-naped tern also breed and there is a huge roost of Lesser and Greater frigatebirds.
Both Bijoutier and St François meet IBA criteria for four species of congregatory water-birds: Black-naped Tern, Saunders’ Tern (Sterna saundersi), Crab Plover (Dromas ardeola) and Ruddy Turnstone (Arenaria interpres). D'Arros is an IBA due to an introduced population of the Seychelles Fody. Other land birds also include the Madagascar turtle dove. There is a strong case to extend the IBA to St Joseph, which has the largest population of Roseate terns (about 300 pairs) and Wedge-tailed shearwaters (about 300,000 pairs) in the outer islands. Black-naped tern also breed and there is a huge roost of Lesser and Greater frigatebirds.
