Size and depth of cold-water coral colonies on 3D reconstructions in the Lacaze-Duthiers canyon, CALADU_2019

Coral assemblages were reconstructed in 3 dimensions (photogrammetry) using the 'Structure from motion' technique, at several locations in the canyon. The two colonial species, M. oculata and D. pertusum, were located on georeferenced photogrammetric models, with geographic coordinates and depths. The size distribution of both species was measured on photogrammetric models.
Population size structure
Number of coral colonies measured = 5387 (COVID containment period) of which
- 851 D. pertusum colonies
- 4536 colonies of M. oculata
Average size :
- D. pertusum = 28 cm high (standard deviation = 15 cm)
- M. oculata = 18 cm (standard deviation = 7 cm)
Maximum sizes recorded on dives 143-05 and 145-07 at around 330 m depth:
- D. pertusum = 88 cm
- M. oculata =50 cm
Length of large colony bushes :
- D. pertusum = up to 2.5 m
- M. oculata = up to 1.8 m
Both species showed differences in average size depending on the sites explored (Fig. 6). For D. pertusum, dives 143-05 and 145-07 revealed larger populations with three distinct size groups for dive 143-05, and two groups for 145-07. At these sites, 67% of colonies averaged between 28 and 52 cm. In contrast, dives 142-04 and 144-06 revealed only small colonies, with an average size of 16 cm.  For M. oculata, the size distribution was similar between sites. The majority of colonies (53%) averaged 18 cm in size, while the other groups averaged 12 cm (26%) and 27 cm (21%). The population structure was homogeneous between sites, with two main groups of 15 cm (70%) and 23 cm (30%).