Features

Finding a place among the fishes

SOSF D’Arros Research Centre

Clare Keating Daly

Words by Clare Keating Daly

Cover image

Dr Ryan Daly sinks onto a bed of sea grass to photograph an anemone fish as part of a rapid biodiversity assessment around D'Arros Island and St Joseph Atoll.
Photo by Guy Stevens

Place is a slippery thing. Read a guidebook, zoom in on a map, search a tag or a keyword, walk its shores or its roads – there are many ways to feel as though we know a place. And yet, a place’s essence, its contents and components remain concealed, unknown. As wilderness, wildness and the ability to find solitude in the natural world disappear before our eyes, it can feel as though there is little left to discover, that the book on nature can be closed. But there is still so much to learn, to record, to preserve. There is so much at risk of slipping away, of remaining unknown.

Our planet harbours unimaginable biodiversity, an estimated 8.7 million species. Today, researchers believe that roughly 85% of these species have not yet been described. Which is to say, we still have a long way to go. We have much to learn about what kind of place this planet is, and the best way to begin to understand this is to act locally.

A constellation of islands scattered some 1,500 kilometres (930 miles) off the coast of eastern Africa, the Seychelles is, in a way, surrounded by fishes. Nonetheless, marine fishes are not a hot topic of study in the country and even less so in the Outer Islands of the Amirantes Archipelago, home of the SOSF-D’Arros Research Centre. There is wilderness here, wildness under the waves. There is much that remains unknown. Thus to understand more about the biodiversity of the Seychelles, what better place to start than with its fishes.

Although the research centre’s reef fish monitoring programme is entering its eighth year, knowledge of the local fishes was limited to a record of the species encountered during general research activities, a list containing approximately 220 names. Suspecting that more fish species could be found in the diverse range of marine habitats, in May 2017 a team of three researchers set out to establish a benchmark.

The team consisted of SOSF-D’Arros Research Centre’s research director Dr Ryan Daly, consultant Dr Guy Stevens and research assistant Justin Blake. Free diving and on scuba, they armed themselves with cameras and clipboards in the field and Fishbase.org and identification guides in the lab. Over 19 days, spending 84 hours underwater, they undertook a rapid biodiversity assessment of the coral reefs and associated habitats found around D’Arros Island and St Joseph Atoll.

Roving through meadows of sea grass and across sand flats, shining a light into the darkest caves and exploring the murky waters of the atoll lagoon, the team more than doubled the number of known fish species in the waters surrounding the island and the atoll. They recorded 514 reef-associated fish species in 71 families, photographing 73% of these records for positive identification.

Cover image

Dr Ryan Daly sinks onto a bed of sea grass to photograph an anemone fish as part of a rapid biodiversity assessment around D'Arros Island and St Joseph Atoll.
Photo by Guy Stevens

Clare Keating Daly Lab Manager

Delving into the wealth of life secreted away in D’Arros waters means spending significant time submerged. Dr Ryan Daly photographs marine life in what amounted to 84 hours underwater.
Photo by Justin Blake

Fishbase, the global biodiversity information system on fishes, currently lists 887 native or endemic reef-associated fish species in the Seychelles. Based on these numbers, the waters surrounding D’Arros and St Joseph support 58% of known reef-associated species recorded in the Seychelles to date. Thus, in under three weeks, the rapid biodiversity assessment team quantified the importance of D’Arros Island and St Joseph Atoll for fish life in the Seychelles. At the start, they thought they were asking the question, ‘What fishes are here?’ By the end, the question they had answered was, ‘What kind of place is this?’

This is a place of refuge. Of the 29 vulnerable or endangered fish species on the IUCN Red List of Threatened Species for the Seychelles, more than half (15) find sanctuary in the diverse habitats that the island–atoll complex provides. For these vulnerable and endangered species, two features stand out as particularly important – in fact, unique in the Amirantes and potentially unique in the Seychelles as a whole.

The first is the system of habitats within St Joseph Atoll. At every low tide, the inner habitats of the atoll are entirely cut off from the surrounding ocean, creating a sort of marine island. The lagoon and flats within this system shape an irreplaceable nursery and foraging area for at least 10 of the 15 Red Listed species recorded during the assessment.

The second is the deep channel that separates D’Arros Island and St Joseph Atoll, the slopes of which plunge to depths of more than 70 metres (230 feet) in less than one kilometre (half a mile). Currents scour the divide, creating some of the deepest waters on the Amirantes Bank. Both the reef manta ray Manta alfredi and the Napoleon wrasse Cheilinus undulatus, two of the four CITES Appendix II-listed species recorded during the assessment, rely on the structure of this distinct channel. The combination of an upwelling of zooplankton and favourable currents to concentrate the zooplankton in the shallows may be one of the reasons manta rays congregate here nearly year-round (read more about the manta rays of D’Arros). Meanwhile, the caverns and caves lining the steep slopes of the channel provide important habitat for Napoleon wrasse. During the assessment, the team watched as up to 11 individuals of this species gathered in the channel before each slipped into a cave to roost safely for the night.

Of course, a place of refuge means a home for Seychelles endemics as well. The black-eye emperor Lethrinus enigmaticus, one of the most abundant species found in the survey, is also commonly sighted in the channel as well as along its slopes and in deep sea-grass meadows.

Rapid Marine Fish Biodiversity Assessment<br />
Artwork by Loris F. Alessandria

Rapid Marine Fish Biodiversity Assessment
Artwork by Loris F. Alessandria

It’s also a place of discovery. The waters surrounding the D’Arros and St Joseph complex are a marine frontier. Spending the equivalent of three and a half days underwater, the rapid biodiversity assessment team often encountered a species thought to be rare and recognised at least three others that were far from home, translating to significant range expansions for these species.

The green sea-grass wrasse Pseudojuloides argyreogaster, a shy, schooling species, was one such discovery. Until this assessment, this low-key wrasse was thought to be rare in its known locations, which were limited to scattered localities in the Seychelles and coastal Tanzania, and photos of it were exceedingly rare. To their delight, the team members often saw – and photographed – the green sea-grass wrasse in the thick meadows of its namesake along reef crests of D’Arros and St Joseph. What’s more, they recorded this understated wrasse at a depth of 25 metres (82 feet), expanding its known depth range from its previous record of a mere six metres (20 feet).

The team made further discoveries at the depths where the green sea-grass wrasse had been thought not to venture. Another rare fish, the blue-lined flasher wrasse Paracheilinus attenuatus was previously known only from St François Island in the Amirantes and through the aquarium trade in Kenya. Here it was present on deep sandy slopes.

There are rare fish and then there are lost fish. Or perhaps not lost, only never noticed. The sightings of three species contributed to significant range extensions during the assessment, collectively adding 11,000 kilometres (6,850 miles) to the recorded range of these species. Diving along the steep reef slopes of the channel between D’Arros and St Joseph, the team recorded a glitter of purple- cheek wrasse Pseudocoris petila during the assessment. Photographed and confirmed, this species was previously known only from the Andaman Islands, some 5,000 kilometres (3,100 miles) away from the Amirantes.

Also known previously only from the Andamans, the blemished razorfish Iniistius naevus was reliably recorded on the deep sand flats just off the D’Arros Island dive centre, adding another 5,000 kilometres of range extension. The frequency at which we’ve sighted these razorfish after the assessment suggests that indeed they are not lost, and that their range may be much larger than originally thought. The final range extension, of 1,000 kilometres (620 miles), was courtesy of a single blue-lined triggerfish Xanthichthys caeruleolineatus making its way along the deep channel between D’Arros and St Joseph. Although it gave only a fleeting view, the fish was photographed and added to the ID catalogue.

Rapid Marine Fish Biodiversity Assessment
Artwork by Loris F. Alessandria

The green sea-grass wrasse was thought to be rare and limited in its range. The D'Arros team discovered this shy fish in the sea-grass meadows of D'Arros Island and St Joseph Atoll, sometimes at a depth of 25 metres – four times deeper than previously recorded.
Photo by Ryan Daly

Like any wild place, the D’Arros–St Joseph complex is one that keeps its secrets well. The assessment team uncovered treasure in the form of four cryptic fish species, all new records for the Seychelles. These tiny gems, tucked away in caves and coral, are potentially common, but their cryptic nature, rather than actual rarity, is probably what accounts for the lack of previous records for them in the Seychelles. In a place so full of life, looking just a little closer reaps great rewards.

A single cave concealed three of the four cryptic species recorded during the assessment. Each was subsequently found in other caves and reef overhangs, but coming upon them in one cave gave the divers the feeling that they’d stumbled into a secret lair. The species in question were the broad-banded pipefish Dunckerocampus boylei, the cave pygmy goby Trimma anaima and the flame pygmy goby T. macrophthalmum. The fourth cryptic species new to Seychelles was positively identified as the blue eye-lined coral goby Gobiodon bilineatus. Once its hiding spot had been found, the team often encountered this brilliant goby crouching in colonies of Acropora corals.

D’Arros Island and St Joseph Atoll are without doubt a place of refuge, discovery and secrets and their ecosystems still leave much to the imagination. However, the results from the SOSF D’Arros Research Centre’s rapid biodiversity assessment of fish communities have not only established the intended benchmark, but also developed the largest reef-associated fish species list for the Seychelles, a country surrounded by fish. Detailed, quantifiable data on fish biodiversity such as these provide the backbone for qualitative interpretation; together, D’Arros and St Joseph are truly a special place.

Like many rapid biodiversity assessments, this undertaking answered more than it set out to achieve. The sheer diversity of fishes – 514 species in 71 families – is astounding. In particular, the final number of species found during the assessment is 182 more than a recent fish survey recorded at World Heritage Site Aldabra Atoll, which is famous for its biodiversity. And yet the team members share a feeling that there is still much to discover here in the habitats of D’Arros Island and St Joseph Atoll. Besides the opportunity to expand known ranges and perhaps find fish that are new to science, there is also the chance to increase our limited knowledge about the Seychelles’ coral and invertebrate species; a reef-wide assessment of qualitative data to add weight to the feeling of wonder that this place already inspires.