Access the data:
- substrate and invertebrates data will soon be available.
The sampling sites have been focused on the spurs around 12m depth. Eight UE (Unité d'Échantillonnage, Sampling Units) have been delimited, each 25m long. These UE are coupled in pairs (1/2, 3/4, 5/6 and 7/8) and five strings are needed to delimit each pair (25m long strings, all parallel to each other, 2m apart for each UE). These units have been sampled every year since 1991 on the outer slope.
The eight sampling units on the outer slope of Tiahura
The eight sampling units on the outer slope of Tiahura (Drone @YC)
Point intersect transects (PIT)
This method is used since 1991 to determine the substrate nature (algae, hard coral, sand, rubble and pavement). On the central string separating to paired UE, the nature of the substrate is noted every 25cm. No lead weighted string is needed, contrary to what is done on the barrier reef since the height between the string and the substrate is way lower on the outer slope (less than 30cm and sometimes nil when the string rest on the coral colonies). The number of replicates is 4.
|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.|
In each belt of the 4 units, the following organisms are sampled:
- Tridacna maxima (small giant clam)
- Turbo marmoratus (great green (marbled) turban)
- Trochus niloticus (commercial top shell)
- Acanthaster planci (crown of thorns starfish)
- all the sea cucumber (mainly on the outer slope Thelenota ananas, Bohadshia argus)
- the free hard corals (Sandalolitha sp, Fungia sp, Herpolitha sp)
- some rare (in census) species (ex : Lambis truncata (seven fingers) ou Charonia tritonis (giant triton))
Line intercept transect (LIT)
All the coral colonies intercepted by a string are accounted for. By pair of units, the colonies are sampled over a total string length of 125m (5 transects of 25m). The number of replicates is twenty.
This techniques is used to sample the fish every year since 1991 and the molluscs and echinoderms since 1993. Each sampling unit matches a quadrat of 100 m² (4x25 m). The number of replicates is eight. However for methodological reasons, each quadrat is divided in two sub-quadrats (2x25 m) to count the fish and four sub-quadrats to count the molluscs and the echinoderms. The data of two juxtaposed sub-quadrats are cumulated for statistical analysis.
Counting the entire fish population is achieved using two passes on the same 50 m² sub-quadrat. On the first pass, a stop every ten meters including at the beginning of the sub-quadrat is made to count the fleeing species (scaridae, labridae, balistidae, acanthuridae...). Then while swimming slowly, the sedentary and cryptic species (serranidae, pomacentridae, holocentridae,...) are accounted for. Some more abundant species are not counted individually but estimated by small groups ("Pterocaesio tile", "Chromis iomelas", "Chromis vanderbilti", "Dascyllus flavicaudus",...). As for the barrier reef, the abundant species "Ctenochaetus striatus" is counted an a second pass while swimming back.