true

ISRA FACTSHEETS

WESTERN INDIAN OCEAN REGION

ISRA FACTSHEETS

WESTERN INDIAN OCEAN REGION

Alphonse Atoll ISRA

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Alphonse Atoll ISRA

Alphonse Atoll

Summary

Alphonse Atoll is located 400 km southwest from the inner islands of the Mahé plateau in Seychelles. The area consists of an atoll with well-defined and extensive reef flats dominated by seagrass and fine sand. The lagoon is enclosed at low tide, but accessible by a channel which greatly influences the hydrodynamics around the atoll. This area sits within a marine protected area and within the Mahé, Alphonse and Amirantes Plateau Ecologically or Biologically Significant Marine Area. It also overlaps with Alphonse Island and Lagoon Key Biodiversity Area. Within this area there are: threatened species (e.g., Grey Reef Shark Carcharhinus amblyrhynchos); reproductive areas (Spotted Eagle Ray Aetobatus ocellatus); and undefined aggregations (Grey Reef Shark).

Alphonse Atoll

Alphonse Atoll

DESCRIPTION OF HABITAT

Alphonse Atoll lies at the southern extremity of the Amirantes archipelago, southwest of Mahé, the main island of Seychelles. Alphonse has a single chamber lagoon with mixed sand, seagrass, and scattered patches of coral (Hamylton et al. 2012). There is continuous coral reef on both the inside rim of the lagoon (on the eastern side) and outer reefs, which are steeply sloping. The Alphonse channel is ~60 m wide, 1 km long, and 5–6 m deep. There is extensive coral growth inside the channel demonstrating a lack of disturbance for +30 years (Islands Conservation Society [ICS] unpubl. data 2023). Tidal movements of water spill over the flats that surround both the island and lagoon, however, these all dry out at low tide (Hamylton et al. 2012). The southeastern corner of Alphonse Atoll has a small shallow (0.5–1 m deep) sandy bay that is sheltered from the strong currents that flow over the flats during tidal movements (ICS unpubl. data 2023). The southern part of Alphonse Island which opens to the lagoon is also sheltered from currents and the reef flat adjacent to the coral reef has more fine sand and silt than other areas.

The area sits within a marine protected area and within the Mahé, Alphonse and Amirantes Plateau Ecologically or Biologically Significant Marine Area (EBSA; CBD 2023). It also overlaps with Alphonse Island and Lagoon Key Biodiversity Area (KBA 2023).

This important Shark and Ray Area is benthopelagic and delineated from inshore and surface waters (0 m) to 60 m based on the bathymetry of the area and the observations of the Qualifying Species in the area.

CRITERION A

VULNERABILITY

Two Qualifying Species considered threatened with extinction according to the IUCN Red List of Threatened SpeciesTM regularly occur in the area. These are the Endangered Grey Reef Shark (Simpfendorfer et al. 2020) and Spotted Eagle Ray (Finucci et al. submitted).

CRITERION C

SUB-CRITERION C1 – REPRODUCTIVE AREAS

Alphonse Atoll is an important reproductive area for one ray species.

Courtship behaviour including ‘mating trains’ lasting multiple days and active biting/mating attempts of Spotted Eagle Rays have been recorded. Animals are seen in aggregations of one female and up to six males in shallow water (0.5–2 m) areas of the Alphonse lagoon, adjacent to the beaches on the southern and southeastern side. These courtship displays are a regular annual event and although sightings data on when they occur are sporadic, accounts vary from February to May (ICS unpubl. data 2012–2022). Additionally, neonate Spotted Eagle Rays (~25–35 cm disc width [DW]) are regularly observed in very shallow water along the beaches of Alphonse Atoll (ICS unpubl. data 2012–2022). The size-at-birth for this species is 33–36 cm DW (Last et al. 2016).

CRITERION C

SUB-CRITERION C5 – UNDEFINED AGGREGATIONS

Alphonse Atoll is an important area of undefined aggregations for one shark species.

Scientific divers have recorded aggregations of adult Grey Reef Sharks 13 times on the western side of Alphonse Atoll at a specific location (from ‘Galawa’ to ‘Bluewater Stop/Wonderland’) from 2017 to 2023 and from January to September (E Brighton unpubl. data 2023; Alphonse Dive log 2017–2023). Four to 12 individuals were seen on each occasion, and animals were of varying sizes (~120–210 cm total length). The function of these aggregations is currently unknown. However, the location where the sharks aggregate is a steep sloping reef with distinct spur and groove structures in the shallows, high levels of live hard coral cover on the slope, and large sea fans below 16 m (ICS 2018). This site experiences cold water upwelling which does not seem to be consistent with the seasonal upwelling around other areas of the atoll and could be an influencing factor. Furthermore, there are often strong currents along this reef (E Brighton pers. obs. 2020–2023).

Alphonse Atoll

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