ATOLL RESEARCH BULLETIN
NO. 577
TERRESTRIAL AND MARINE ECOLOGY OF ETOILE,
AMIRANTES, SEYCHELLES
BY
ANNELISE B. HAGAN, THOMAS SPENCER, JENNIFER ASHWORTH,
JUDE BIJOUX, RODNEY QUATRE, MARTIN CALLOW, AND BEN STOBART
ISSUED BY
NATIONAL MUSEUM OF NATURAL HISTORY
SMITHSONIAN INSTITUTION
WASHINGTON, D.C., U.S.A.
MARCH 2010
Figure 1. Location of beach profiles (EB1, EB2, EB3 and EB4) and shallow-water transects (ESW1,
ESW2, ESW3 and ESW4) at Etoile, 25th January 2005. Habitat map from Spencer et al. (2009).
TERRESTRIAL AND MARINE ECOLOGY OF ETOILE,
AMIRANTES, SEYCHELLES
BY
ANNELISE B. HAGAN,1 THOMAS SPENCER,1 JENNIFER ASHWORTH,1
JUDE BIJOUX,2 RODNEY QUATRE,2 MARTIN CALLOW,3 AND BEN STOBART1
INTRODUCTION
The Amirantes group, Seychelles, comprises 24 islands and islets lying between
5° and 6° south of the equator on the Amirantes Bank, western Indian Ocean. The
islands were discovered by the Portuguese navigator Vasco de Gama on his second
voyage to India in 1502, soon after acceding to the rank of Admiral, and the islands were
subsequently named Ilhas do Almirante or Admiral’s Islands (Lionnet, 1970). The group
extends over a distance of 138 km, from African Banks in the north to Desnoeufs in the
south. Etoile is one of the two sand cays which rises up from the Banc de la Boudeuse,
approximately 30 km south-west of Poivre atoll. The other sand cay, Boudeuse, lies 30
km southwest of Etoile. Etoile is believed to have been named by Chevalier du Roslan in
1771, after one of the two ships of Bougainville’s round the world voyage from 1766 to
1769. Due to its small size (and thus its probable lack of economically significant guano
deposits), the fact that the sea is reportedly often rough around this shallow bank and the
difficulty of getting ashore, the island has never been inhabited. It is designated as a bird
reserve.
A collaborative expedition between Khaled bin Sultan Living Oceans Foundation,
Cambridge Coastal Research Unit and Seychelles Centre for Marine Research and
Technology – Marine Parks Authority to the southern Seychelles was conducted onboard
M.Y. Golden Shadow, from 10th – 28th January 2005. The primary aim of the expedition
was to use a CASI (Compact Airborne Spectrographic Imager) sensor onboard a seaplane
to conduct large-scale mapping of the southern Amirantes, Alphonse/St. François
(Spencer et al., 2009) and Providence Bank. All surveys at Etoile were conducted on 25th
January 2005.
__________________________________________________
Cambridge Coastal Research Unit, Department of Geography, University of Cambridge, Cambridge, CB2
3EN, UK.
2
Seychelles Centre for Marine Research and Technology – Marine Parks Authority,
Victoria, Mahé, Seychelles.
3
The Trident Trust, The Smokehouse, Smokehouse Yard, 44-46 St John Street, London, EC1M 4DF, UK.
1
Manuscript received 15 October 2009.
2
REGIONAL BATHYMETRY, TOPOGRAPHY AND VEGETATION
Etoile is unusually situated near the western margin of the Amirantes Bank (most
islands in the Amirantes sit on the eastern margin). Water depths immediately to the west
of Etoile rapidly exceed 1,000 m but the western margin of the Amirantes Bank reaches
to within 11 to 17 m of sea level, compared to depths in excess of 50 m in the centre of
the Bank to the east. Etoile is positioned at a location where this marginal rim doubles in
width; it is likely that the island occupies a small topographic high as local water depths
are 9 m or less.
The sand cay is small (1 ha), broadly crescentic in form and flanked by an area of
reef-flat sands, 600 m wide and bounded by a zone of breaking waves, to the southeast
(Fig. 1, Plate 1). Both the cay and the reef-flat sands sit centrally within a larger, ovalshaped fore-reef sandsheet, ca. 1.5 km long and 1 km wide, orientated NW-SE. A tongue
of bare sand also extends to the north, reaching 1.6 km from the sand cay. The centre of
the cay supports a mat of herbaceous vegetation (Plate 2).
TERRESTRIAL SURVEY METHODS
Flora and Fauna Surveys
No quantitative vegetation surveys were undertaken at Etoile. However,
general observations of the bird-life and plant-life present were recorded. If birds and
plants could not be identified in situ, digital photographs were taken for subsequent
identification by local experts on Mahé.
Beach Surveys
Four beach profiles were measured at Etoile on the north (EB1; 5o53.095′S,
53o01.624′E - 5o53.068′S, 53o01.628′E), east (EB2; 5o53.102′S, 53o01.642′E - 5o53.117′S,
53o01.693′E) south (EB3; 5o53.115′S, 53o01.630′E - 5o53.152′S, 53o01.639′E) and west
(EB4; 5o53.107′S, 53o01.624′E - 5o53.144′S, 53o01.548′E) sides of the island (Fig. 1).
Profiles were measured by Abney level and tape, in an offshore direction perpendicular
to the beach, beginning at the terrestrial vegetation line and continuing to the offshore
step (where the waves were breaking, typically marked by a downward step) or as far as
safely possible into the water. Eight surface scrape sediment samples, of ca. 150 – 350 g
by weight, were collected, typically at the start (upper beach) and end (lower beach) of
each profile. Positions were recorded for the start and end of each beach profile and for
the sites of the sediment samples using a hand-held GPS unit (horizontal resolution = ±10
m). Sediments were dried, disaggregated and sieved using standard techniques at 0.25 phi
intervals.
3
MARINE SURVEY METHODS
Shallow-water Boat Transects
Shallow-water transects were undertaken using a rigid inflatable boat at four
sites around Etoile. Transects started at a water depth of approximately 20 m (the
limit at which the bottom substrate could be accurately determined from the surface)
and ran in towards a pre-decided point on the cay surface. Transects ran from N-S
(ESW1; 5o52.646′S, 53o01.379′E - 5o52.894′S, 53o02.231′E), E-W (ESW2; 5o52.891′S,
53o02.231′E - 5o53.024′S, 53o01.730′E), S-N (ESW3; 5o53.937′S, 53o02.015′E 5o53.441′S, 53o01.808′E) and W-E (ESW4; 5o53.182′S, 53o00.945′E - 5o53.218′S,
53o01.530′E) (Fig. 1). Each time the boat was stopped a GPS position was taken and the
water depth and bottom substrate (viewed through a glass-bottomed bucket) recorded.
Nine substrate observations were recorded on the N-S transect, 13 substrate observations
were recorded on the W-E transect, 14 substrate observations were recorded on the S-N
transect and 15 substrate observations were recorded on the E-W transect. No SCUBA
diving surveys were undertaken at Etoile due to time constraints.
RESULTS OF TERRESTRIAL SURVEYS
Flora and Fauna Surveys
The herbaceous vegetation mat consists of a mixture of Portulaca oleracea and
Boerhavia spp. (including Boerhavia repens). Numerous nesting seabirds were associated
with the vegetated area in January 2005 (Plate 3). On the southern side of the vegetation
patch, there were two separate colonies (n = 120 and n = 140) of Anous stolidus (Brown
or Common Noddy) and on the northern side, there was one colony (n = 60) of Sterna
fuscata (Sooty Tern) and a mixed colony of Anous stolidus (Brown or Common Noddy)
(n = 80) and Sterna fuscata (Sooty Tern) (n = 50). Numerous eggs and chicks were
observed in these colonies.
Beach Surveys
Beach sediments at the cay on Etoile are all coarse sands, generally well sorted
but ranging from moderately well sorted to very well sorted in character (Table 1, Fig. 2).
All beach profiles fall within the textural group of slightly gravelly sand, except for the
base of profile EB2 (i.e. sample EB2.2) which is classified as sand.
Sample
Environment
D50
Mz
σ1
SK1
KG
EB1.1
Upper beach
0.514
0.517 0.447 -0.013 0.998
Coarse Sand, Well Sorted,
Symmetrical, Mesokurtic
EB1.2
Lower beach
0.644
0.626 0.429 -0.128 1.018
Coarse Sand, Well Sorted,
Coarse Skewed, Mesokurtic
EB2.1
Upper beach
0.524
0.520 0.384 -0.031 1.073
Coarse Sand, Well Sorted,
Symmetrical, Mesokurtic
EB2.2
Lower beach
0.774 0.786 0.329
EB3.1
Upper beach
0.442 0.421 0.366 -0.016 1.072
Coarse Sand, Well Sorted,
Coarse Skewed, Mesokurtic
EB3.2
Lower beach
0.436
0.427 0.319
Coarse Sand, Very Well Sorted,
Symmetrical, Mesokurtic
EB4.1
Upper beach
0.317
0.288 0.560 -0.107 1.026
EB4.2
Lower beach
0.668 0.689 0.382
0.032 0.962
-0.002 1.038
0.018 0.952
Description
Coarse Sand, Very Well Sorted,
Symmetrical, Mesokurtic
Coarse Sand, Moderately Well Sorted,
Coarse Skewed, Mesokurtic
Coarse Sand, Well Sorted,
Symmetrical, Mesokurtic
4
Table 1. Folk and Ward (1957) particle size distribution statistics for sediment samples from beach profiles. Units are
given on the phi (ø) scale.
5
Figure 2. Cumulative frequency curves for sediment samples from upper and lower beaches.
Beaches vary in width from ca. 45 m, on the north side of the cay (Fig. 3a), to
70 m on the southern aspect (Fig. 3c), where the beach lies behind the 600 m wide reef
flat. Widths are ca. 60 m on east and west-facing beaches (Figs. 3b, 3d). The elevation
of the top of the beach generally falls within the range of + 2.5 to + 2.9 m above mean
sea level but reaches + 3.7 m on the windward eastern side of the sand cay. Beach
angles are typically 4 – 6o on the lower beach and 1 – 3o at the upper margin where field
observations suggested that profiles are flattened by wave washover processes (Plate 4).
In the absence of washover, the steepest beaches lie on the northern side of the cay, the
area most protected from waves from the south-east, being 6 – 9o on the lower beach and
reaching 14 o at the landward beach margin (Fig. 3a).
6
Figure 3. Beach profile a) on the north side of the island at 05o53.095′S, 53o01.624′E - 5o53.068′S,
53o01.628′E; b) on the east side of the island at 5o53.102′S, 53o01.642′E - 5o53.117′S, 53o01.693′E; c) on
the south side of the island at 5o53.115′S, 53o01.630′E - 5o53.152′S, 53o01.639′E; d) on the west side of the
island at 5o53.107′S, 53o01.624′E - 5o53.144′S, 53o01.548′E.
7
RESULTS OF MARINE SURVEYS
Shallow-water transects
Benthic observations from a depth of 13.5 m to 3.5 m were made on the north side
of the island over a distance of approximately 0.5 km. In depths greater than 10 m, dense
seagrass beds were observed, interspersed with coral rubble. Habitat mapping has shown
large patches (long axes of > 200 m) of medium density seagrass within these dense
seagrass beds (Spencer et al., 2009). At shallower depths, the substrate was dominated by
a mixture of sand, coral rock and rubble. Minimal live coral (< 5% cover) was observed
here. On the eastern side of the island, the shallow water transect was conducted between
depths of 18 m and 2.6 m over a distance of approximately 0.75 km. At depths greater
than 16 m, and between 10 m and 4 m depths, Thalassodendron ciliatum seagrass was
interspersed with sand and rubble. Habitat mapping shows seagrass beds here organised
into linear structures, separated by sand channels at typical wavelengths of ca. 60 m
(Spencer et al., 2009). Between 10 m and 16 m depths, the substrate was comprised of
coral rubble on sand and at 2.6 m, bare rock pavement was observed. No live coral was
observed on this transect. The benthic habitat on the south side of the island was again
dominated by Thalassodendron ciliatum seagrass beds at depths greater than 11 m, with
circular areas (typical diameter of 100 – 180 m, but up to d = 320 m) of medium density
seagrass; shallower than 10 m the benthos was rock pavement, covered in filamentous
algae. On the western side of the island, Thalassodendron ciliatum seagrass was observed
from ca. 20 m up to 5 m water depth; linear structures interspersed with sand were often
observed, with wavelengths, as on the eastern side of the island, at 60 m. Bare rock
pavement was seen at shallower depths.
DISCUSSION
The sand cay at Etoile is an unstable system. In 1976 the island was reported
to have been ‘largely eroded away, leaving a large sea-swept sand bank with a small,
ca. 0.3 ha vegetated area’ (Feare et al., 2007: 428) and the comparison of aerial
photographs from 1999 and airborne remote sensing from 2005 shows considerable
change in size, orientation and position (Spencer et al., 2009). This dynamism has not
allowed the development of an extensive vegetative cover and has implications for
seabird populations. The breeding colony of Sula dactylatra (Masked Booby) reported
from Etoile in September – October 1941 (Vesey Fitzgerald, 1941) was not present in
November 1976; it has been suggested that the extinction of this colony was due to
significant island erosion (Feare, 1978). Additionally, the greatest threat to the breeding
colony of Sterna fuscata has also been identified as island erosion (Feare et al., 2007).
The shallow-water surveys around the Etoile sand cay indicate that there is no true
coral reef present. The benthos is dominated by dense Thalassodendron ciliatum seagrass
beds, which reaches as shallow as 5 m water depth. On the east, south and west sides of
the island, no coral was observed, only bare rock pavement, with an occasional covering
8
of filamentous algae. Live coral cover was observed at only one sampling location, on
the northern side of the island, and here was estimated to represent less than 5% of the
benthos. The coral bleaching event of 1997-98 that severely impacted the reefs of the
granitic Seychelles (Lindén and Sporrong, 1999; Spencer et al., 2000) is unlikely to have
caused any damage at Etoile. It appears that there is little coral present at Etoile and what
is present can be best described as a patchy cover of coral communities rather than a true
reef structure.
ACKNOWLEDGEMENTS
Observations in the Republic of Seychelles were supported through a
collaborative expedition between Khaled bin Sultan Living Oceans Foundation,
Cambridge Coastal Research Unit, University of Cambridge and Seychelles Centre for
Marine Research and Technology – Marine Parks Authority (SCMRT–MPA). The authors
would like to acknowledge Prince Khaled bin Sultan for his generous financial support
of the expedition and use of the M.Y. Golden Shadow and Capt. P. Renaud, Executive
Director, Khaled bin Sultan Living Oceans Foundation for extensive logistical support.
We also graciously acknowledge the encouragement, collaboration, and logistical support
provided by the Seychelles Government and thank the Island Development Company,
Seychelles, for permission to visit islands in the southern Seychelles. Laboratory analyses
for the particle size distributions of beach sediments were undertaken by Chris Rolfe,
Senior Laboratory Technician, Department of Geography, University of Cambridge;
statistics were obtained through the ‘Gradistat’ package (© S. Blott). We are extremely
grateful to Adrian Skerrett for discussions over seabird colonies at Etoile and to Sarah
Hamylton for providing Figure 1.
REFERENCES
Feare, C.J.
1978. The decline of Booby (Sulidae) populations in the western Indian Ocean.
Biological Conservation 14:295-305.
Feare, C.J., S. Jaquemet and M. Le Corre
2007. An inventory of Sooty Terns (Sterna fuscata) in the western Indian Ocean with
special reference to threats and trends. Ostrich 78(2):423-434.
Folk, R.L. and W.C. Ward
1957. Brazos River bar: a study in the significance of grain size parameters.
Journal of Sedimentary Petrology 27:3-26.
Lindén, O. and N. Sporrong
1999. Coral Reef Degradation in the Indian Ocean. CORDIO Project Report. FRN/
MISTRA/SIDA/World Bank/WWF Publication. 108p.
Lionnet, J.F.G.
1970. Appendix: Names of the islands. Atoll Research Bulletin 136:221-224.
9
Spencer, T., K.A. Teleki, C. Bradshaw and M.D. Spalding
2000. Coral bleaching in the Southern Seychelles during the 1997-1998 Indian Ocean
warming event. Marine Pollution Bulletin 40:569-586.
Spencer, T., A.B. Hagan, S.M. Hamylton and P. Renaud
2009. The Atlas of the Amirantes Cambridge Coastal Research Unit, University of
Cambridge, Cambridge, UK. vi + 66p.
Vesey-Fitzgerald, D.
1941. Further contributions to the ornithology of the Seychelles islands. Ibis 14(5):518531.
PLATES
11
Plate 1. Aerial view of Etoile showing sand cay, vegetated area and surrounding marine
habitats (source: Maps Geosystems; reproduced with kind permission of the Government
of the Seychelles). (Note: Photograph taken in 1999; shape of cay different from that
shown on the 2005 habitat map).
12
Plate 2. Vegetated area of the sand cay at Etoile (photograph: Jen Ashworth, January
2005).
Plate 3. Sterna fuscata (Sooty Tern) nesting on herbaceous vegetation mat at Etoile
(photograph: Jen Ashworth, January 2005).
13
Plate 4. Unvegetated area of the sand cay at Etoile, showing a wide berm at the landward
margin of the beach and sand washover deposits (photograph: Jen Ashworth, January
2005).