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 550m of depth. The presence of gorgonian forests and deep-sea sponge aggregations in Le Danois Bank was the cause of its declaration as El Cachucho' Marine Protected Area (MPA), being included in the Natura 2000 network. The Placogorgia sp. is a structuring species of the deep Cantabrian Sea; parameters such as population structure and morphology inform on the overall health of this vulnerable habitat, but the estimation of gorgonian metrics often requires destructive sampling. The use of non-invasive methodology, which does not cause damage or alterations on benthic communities, is particularly necessary in vulnerable ecosystem studies and Marine Protected Areas (MPA) monitoring. This study proposes a semi-automatic methodology to assess gorgonian morphometries fitting planes to colonies. Video transects acquired in Le Danois Bank, during the ECOMARG-2017 survey using the Politolana underwater towed vehicle were used. 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 digitized surfaces manually is validated (R2Combining double low line0.97). The results show that gorgonian population was mostly dominated by small colonies. The population structure distribution shows a high proportion (?22%) of recruits (<0.05m 2 ) of fan surface.

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