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A 100m by 200m sampling zone (longest length parallel to the reef crest) was chosen from an aerial photograph (scale 1/30000, taken in 1986). This zone has been cut in 200 sampling units (unité d'échantillonnage, UE) of 100 m² (2x50m), all parallel to the reef crest. Ten of them numbered UEB1 to UEB10 (Unité d'Échantillonnage Barrière) have been chosen randomly. These units are sampled years after years since 1990.
Point intersect transect (PIT)
The line of each transect is 50m long once tended between two stainless steel stakes. On each line, the distance between two points is 50cm. The nature of the substrate is thus sampled vertically over 100 points per transect. A lead weighted line is used to determine where to sample where it falls bellow the point on the line.
|Rubble (R)||All free hard coral fragment between 0.2 and 15 cm.|
|Sand (S)||Sendiment which fragments of size less than 0.2cm and do not stay in suspension when stirred.|
|Live coral (C)||All living hard coral|
|Turf (T)||All thin turf|
|Coralline encrusting algae (P)||All the coralline encrusting algae|
|Macroalgae (A)||All non coralline algae easily indentified.|
|Other (O)||Any other organism (anemone, shell, waste, soft coral,...).|
|Pavement (D for dalle in French)||All the hard an compact surfaces without encrusting algae nor turf nor macro algae. NB. this category remains scarce.|
These transects are also used as a central line along which benthic organims are sampled along a belt of 1m each side of the line (100m2).
- Tridacna maxima (small giant clam)
- Chama imbricata (bivalve)
- Spondylus sp (bivalve)
- Turbo marmoratus (great green (marbled) turban)
- Trochus niloticus (commercialtop shell)
- Arca ventricosa (bivalve)
- Acanthaster planci (crown of thorn starfish)
- Echinothrix sp (sea urchin)
- Echinometra mathai (sea urchin)
- Diadema sp (sea urchin)
- Halodema atra (sea cucumber)
- Thelenota ananas (pinaple sea cucumber)
- Bohadshia argus (sea cucumber)
- some species rarely appear in census (Cyprea tigris, Lambis truncatus…)
In order to setup each quadrats on the field, lines must be installed for each sampling unit. On both sides of the 50m string, a 2m width belt (1m on each side of the line) is estimated when counting. The molluscs, echinoderms and fish are sampled. The sampled surface is then 100 m² (2x50 m) per quadrat.
On the barrier reef, the fish census is completed by making 2 passes on the same quadrat. On the first pass, every 10m (including the beginning of the transect), a stop is made to count the fleeing species (scaridae, labridae,...). Then while swimming slowly the sedentary and cryptic species are counted (serranidae, pomacentridae, holocentridae,...), except the six-bar wrasse ("Thalassoma hardwicke") and the striated surgeofish ("Ctenochaetus striatus"). On the second pass, these two species are then counted while swimming swiftly back