Seychelles Plateau and Adjacent Oceanic Waters IMMA
Size in Square Kilometres
158 283 km2
Qualifying Species and Criteria
Spinner dolphin – Stenella longirostris
Criterion B (2)
Sperm whale – Physeter macrocephalus
Criterion A
Blue whale – Balaenoptera musculus
Criterion A
Common bottlenose dolphin – Tursiops truncatus
Criterion B (2)
Indo-Pacific bottlenose dolphin – Tursiops aduncus
Criterion B (2)
Marine Mammal Diversity
Criterion D (2)
Balaenoptera musculus, Globicephala macrorhynchus, Grampus griseus, Kogia breviceps, Kogia sima, Megaptera novaeangliae, Mesoplodon densirostris, Orcinus orca, Peponocephala electra, Physeter macrocephalus, Pseudorca crassidens, Stenella attenuata, Stenella coeruleoalba, Stenella longirostris, Tursiops aduncus, Tursiops truncatus, Ziphius cavirostris
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Summary
The Seychelles Plateau includes the islands of Mahé, Alphonse and the Amirantes groups of islands. These mid-ocean shallow banks and islands surrounded by deep waters are highly productive, providing important habitat for fish, seabirds, sea turtles and Manta rays, as well as a high diversity of marine mammals. A 2010 large-scale aerial survey revealed that this area had the second highest levels of cetacean density and diversity of all the areas surveyed in the southwest Indian Ocean. The most abundant species observed in the area are small delphinids (Stenella spp. likely to be spinner and spotted dolphins). However, larger species such as short-finned pilot whales (Globicephala macrorhynchus), Risso’s dolphins (Grampus griseus), sperm whales (Physeter macrocephalus) and pygmy or dwarf sperm whales (Kogia spp.) are also abundant in the area. Endangered Antarctic blue whales (Balaenoptera musculus intermedia) are also encountered seasonally in the area and pygmy blue whales(B. m. brevicauda) are expected to transit the area on migration.
Description of Qualifying Criteria
Criterion A – Species or Population Vulnerability
Since end of the whaling, the vulnerable sperm whale was encountered with a significant occurrence in the area of Amirantes. The area could represent an important area for calves with a relative high calving rate, compares to other areas as Galapagos or Ecuador while smaller aggregation of individuals was reported (Kahn et al., 1993). Endangered pygmy blue whales (Balaenoptera musculus brevicauda) are also found in the area (Mikhalev 2000); however, except for the south of Sri Lanka, no abundance estimates in the Northern Indian Ocean are available for this species (Cooke, 2018). Foetuses found in individuals taken by Soviet whalers north of the Seychelles Plateau, could indicated that the species may give birth in the area, despite size under the prenatal limit (Mikhalev 2000). The Critically Endangered Antarctic blue whales (Balaenoptera musculus intermedia) is known to occur in the area during the austral summer (Anderson et al, 2012). Considering their abundance in the southern hemisphere (<3 000 individuals, Cooke, 2018), the Seychelles likely represents an important area for the species. Criterion B: Distribution and Abundance
Sub-criterion B2: Aggregations
According to the aerial REMMOA survey, small and medium-sized delphinids (spinner dolphins, pantropical spotted dolphins, false killer whales and short finned pilot whales) were most abundant and aggregate in high numbers in the area. In addition, land and sea-based visual monitoring confirmed that common bottlenose dolphins and Indo-Pacific bottlenose dolphins congregate year-round around Alphonse and Desroches islands (pers. com. P-A. Adam, Island Conservation Society).
Criterion D: Special Attributes
Sub-criterion D2: Diversity
The Seychelles Plateau supports an important diversity of marine mammals. Indeed, more than 17 species have been encountered in the area (Van Canneyt et al, 2010; Kiszka, 2015, Webster and Rowat, 2016). Following a large aerial survey of the South West Indian Ocean (REMMOA survey) highest diversity was encountered in the Seychelles blocks (and the North Mozambique Channel) suggesting hotspots of taxonomic richness in both areas (Laran et al, 2017a). This represents more than half of the 28 species of marine mammal that have been recorded within the entire Seychelles waters (Hermans and Pistorius, 2008). Species encountered in the area are common bottlenose dolphin (Tursiops truncatus), Indo-Pacific bottlenose dolphin (Tursiops aduncus), false killer whale (Pseudorca crassidens), Cuvier’s’ beaked whale (Ziphius cavirostris), Blainville’s beaked whale (Mesoplodon densirostris), striped dolphin (Stenella coeruleoalba), Fraser’s dolphin (Lagenodelphis hosei), rough-toothed dolphin (Steno bredanensis), pygmy killer whale (Feresa attenuata) and killer whale (Orcinus orca) occasionally.
Supporting Information
Anderson, C., Branch, T.A., Alagiyawadu, A., Baldwin, R. and Marsac, F. 2012. Seasonal distribution, movements and taxonomic status of blue whales (Balaenoptera musculus) in the northern Indian Ocean. Journal of Cetacean Resources and Management, 12 (2):203-218.
Best, P. B., Rademeyer, R. A., Burton, C., Ljungblad, D., Sekiguchi, K., Shimada, H., Thiele, D., Reeb, D. and Butterworth, D. S. 2003. The abundance of blue whales on the Madagascar Plateau. Journal of Cetacean Research and Management 5(3):253-260
Cooke, J.G. 2018. Balaenoptera musculus. The IUCN Red List of Threatened Species 2018:. http://dx.doi.org/10.2305/IUCN.UK.2018-2.RLTS.T2477A50226195.en
Hermans, A. and Pistorius, P-A. 2008. Marine Mammal Diversity in the Remote Waters of Aldabra Atoll, Southern Seychelles. Atoll Research Bulletin, 564:1-9.
Kahn, B., Whitehead, H. and Dillon, M. 1993. Indications of density-dependent effects from comparisons of sperm whale populations. Marine Ecology progress series, 93:1-7.
Kiszka, J., 2015. ‘Marine Mammals: A review of status, distribution and interaction with fisheries in the Southwest Indian Ocean’. In: R.P. Van der Elst and B.I .Everett (eds.) Offshore fisheries of the Southwest Indian Ocean: their status and the impact on vulnerable species, pp. 305-322. Oceanographic Research Institute, Special Publication, 10.
Laran, S., Authier, M., Van Canneyt, O., Doremus, G., Watremez P. and Ridoux, V. 2017a. A Comprehensive Survey of Pelagic Megafauna: Their Distribution, Densities, and Taxonomic Richness in the Tropical Southwest Indian Ocean. Front. Mar. Sci., 4(139).
Laran S., Dorémus, G., Van Canneyt, O., Ridoux, V. 2017b. Synthèse des campagnes aériennes : REMMOA et SAMM. Rapport pour l’Agence des Aires Marines Protégées. (Synthesis of the aerial survey REMMOA and SAMM. Report for the Marine Protected Area Agency). 75p
Longhurst, A.R. 2007. Ecological geography of the sea. Academic Press.
Mannocci, L., Laran, S., Monestiez, P., Dorémus, G., Van Canneyt, O., Watremez, P., Ridoux, V. 2013. Predicting top predator habitats in the South West Indian Ocean. Ecography 36: 1-18.
Mikhalev, Y.A. 2000. Whaling in the Arabian Sea by the whaling fleets Slava and Sovetskaya Ukraina. Soviet Whaling Data (1949-1979), pp. 141-180. Center for Russian Environmental Policy, Moscow, Russia.
New, A. L., Stansfield, K. Smythe-Wright, D., Smeed, D. A., Evans, A. J. and Alderson, S. G. 2005. Physical and biochemical aspects of the flow across the Mascarene Plateau in the Indian Ocean. – Philosophical Transactions of the Royal Society, 363:151-168.
Samaran, F., Stafford, K.M., Branch, T.A., Gedamke, J., Royer, J.Y., Dziak, R.P. and Guinet, C. 2013. Seasonal and geographic variation of southern Blue Whale subspecies in the Indian Ocean. PLOS One. 8(8): e71561.
Van Canneyt, O., Dorémus, G., Laran, S., Ridoux, V. and Watremez, P. 2010. REMMOA Sud Ouest Océan Indien: Rapport intermédiaire pour l’Agence des Aires Marines Protégées (REMMOA SouthWest Idian Ocean, survey report for the Marine Protected Area Agency), 70 pp.
Webster, I. and Rowat, D. 2016. Conserving marine mammals around Seychelles while maintaining sustainable use of marine resources against a background of urban development, petroleum exploration and climate change. Marine Conservation Society Seychelles. 51pp.
Zemsky VA, Sazhinov EG. 1982. ‘Distribution and current abundance of pygmy blue whales’. In V.A. Arsen’ev (Ed) Marine mammals, pp. 53–70. Moscow, Russia: All-Union Research Institute of Marine Fisheries and Oceanography (in Russian) [Transl. by Gurevich VS in 1994, translation edited by Donahue MA, Brownell Jr RL as Southwest Fisheries Science Center Administrative Report LJ-94-02.
