Semi-automatic method of fan surface assessment to achieve gorgonian population structure in le Danois Bank, Cantabrian Sea

Abstract

This study presents a semi-automatic method to estimate fan surface of a Placogorgia sp. octocoral assemblage using 3D point clouds in El Cachucho MPA at 550 m of depth. The semi-automatic methodology uses fitting planes to assess colonies morphometries. Using Pix4D Mapper Pro and Cloud Compare software, size and morphometry of fan-shaped gorgonians and forest population structure were assessed. RMS of fitting planes shows that the geometrical figure chosen is suitable to retain the morphometric characteristics of the specimens of this species. The adjustment of semi-automatic values with a sample of manually digitized surfaces is validated (R2=0.97). The results show that gorgonian population was mostly dominated by small colonies. The population structure distribution shows a high proportion (~22%) of recruits (< 0.05 m2 ) of fan surface.