The marine environment surrounding D’Arros Island and St Joseph Atoll boasts a diversity of habitats and species. Compared to the other islands in the Amirantes, marine ecosystems are significantly healthier because, over the past four decades, the private owners have done much to preserve habitats, reduce fishing pressure and limit poaching.
The lagoon, which has a maximum depth of 6.5 metres, is divided into a series of basins by parallel flat-topped reef ridges or so-called ‘ribbon reef’. These ridges, the tops of which emerge at low tide, support thick growths of sea grass that are frequently visited by foraging turtles. The bottom is sandy and interspersed with patches of temperature-resistant corals that provide shelter for a host of fishes, crustaceans, turtles and rays. Because sediment is transported over the reef flat on the incoming tide, the water in the lagoon is generally warmer and turbid.
The surrounding reef flats are periodically exposed at low tide, but in some places shallow pools and lower-lying areas are permanently inundated with water. In the south-east of the atoll, where the inner reef flat is protected from wave action and tidal currents, millions of fiddler crabs emerge from their holes at low tide to feed on fresh silt deposits. Using their claws and mouthparts, the crabs work over the entire surface of the reef flat, leaving behind small balls of filtered sand. As soon as the flats are submerged again, stingrays arrive to dig out these crabs and other burrowing creatures. Shark pups and bonefish flood over the flats with the rising tide to hunt for sprats and shrimps. On the high tide, larger sharks arrive in search for rays, bonefish and shark pups.
Where it meets the open ocean, the reef flat is replaced by a coral reef that slopes downward for about 25 metres before abruptly meeting a sandy plateau. Along the northern shores, the reef slope is steep and in many places it is completely covered by corals. In the south, the reef slope is more gradual and corals are more sparse. Apart from corals, other prominent benthic life forms on the reef slope include soft corals, gorgonians, sponges, algae and zoanthids.
The coral reefs have recovered well since the devastating coral bleaching event in 1998, when approximately 70% of the corals in the Seychelles died due to global warming. Coral bleaching occurs when above-average sea temperatures result in the expulsion of the colourful microscopic algae that occur mutualistically in coral skeletons. The loss of these photosynthetic algae turns corals white, diminishes their energy resources and eventually results in their death. Despite their slow growth, corals have made a remarkable recovery at D’Arros and St Joseph and every year thousands of juveniles inundate large areas that were previously devastated. Corals are the building blocks of tropical marine ecosystems and we have noticed a steady increase in macro-invertebrates, fishes, stingrays and even the larger charismatic species, including sharks, manta rays, dolphins and turtles.
Blacktip reef sharks and sicklefin lemon sharks thrive in the shallow lagoon and reef flat habitats of the atoll. Both species breed in the lagoon and shark pups can be seen patrolling the shallows all year round. Other sharks regularly encountered around the islands are white-tip reef, tawny nurse and grey reef. Bull sharks and hammerheads are less common but have been encountered on several occasions. Further out to sea, where the Amirantes bank drops off into the deep, silvertip, silky and oceanic white-tip sharks have been seen.
Hundreds of endangered hawksbill and green turtles nest on the beaches of D’Arros and St Joseph every year. Whereas green turtles emerge to nest at night, hawksbills nest in the daytime, a phenomenon unique to the Seychelles. During the peak hawksbill nesting season in December, several females may be encountered during a short walk along the beach. The nesting populations of both species are closely monitored by staff who patrol the islands almost every day. All nesting tracks are recorded on the GPS and any turtles encountered are carefully measured and tagged. Having been established in 2004 by esteemed turtle biologist Dr Jeanne Mortimer, this is the longest-running turtle monitoring programme in the Amirantes.
The shallow marine environment at D’Arros and St Joseph provides habitats that are crucial to the development of juvenile green and hawksbill turtles, the numbers of which probably range into the thousands. Green turtles graze sea grass on the reef flats at high tide. As the tide drops and the reef flats become exposed, the turtles retreat to the deeper lagoon or the adjoining coral reefs to rest. A leisurely dive along the reef crest at low tide can yield more than 30 turtle sightings. Hawksbills forage in a variety of habitats, seeking out sponges and zoanthids. Their preferred habitat is the shallow, silty region in the south-east of the atoll as it provides an abundance of food. It is a long way to deeper water from there, so turtles are forced to sit out the low tide in small stagnant pools on the reef flats, often tolerating water temperatures that exceed 37 ˚C. These turtles are clearly distinguishable from others, as their lack of contact with the open ocean ensures that they are covered from head to flipper in green algae.
The fish species assemblage numbers more than 300 and includes the entire trophic spectrum of herbivores, corallivores, invertivores and piscivores. Large shoals of beautifully coloured fish of a variety of shapes and sizes swarm over the outer coral reef, occasionally darting about in unison to avoid attacks from predatory fish such as tuna, trevally, snapper and barracuda. In the atoll, bonefish, Indian permit, trevallies and barracuda chase sprats into the shallows, gulping down mouthfuls of them at a time.
Manta rays circle D’Arros Island and St Joseph Atoll year-round, feeding on plankton aggregations as close as 20 metres from the beach. Groups of eagle rays are often encountered patrolling the reef edge and inquisitive stingrays spanning up to two metres in disc-width rise up from the ocean floor to confront divers before darting off.
Bottlenose, spinner and Risso’s dolphins are regularly spotted around the island, while melon-headed whales, shortfin pilot whales and humpbacks are less common.