In-situ growth rate assessment of hexactinellid sponge Asconema setubalense using 3D photogrammetric reconstruction

We describe the first application of a non-invasive and novel approach to estimate the growth rate of Asconema setubalense (Porifera, Hexactinellida) through the use of 3D photogrammetric methodology. Structure-from-Motion techniques (SfM) were applied to videos acquired with the Politolana ROTV in the El Cachucho Marine Protected Area (MPA) (Cantabrian Sea) on three different dates (2014, 2017, and 2019) over six years. With these data, a multi-temporal study was conducted within the framework of MPA monitoring. A complete 3D reconstruction of the deep-sea floor was achieved with Pix4D Mapper Pro software for each date. Having 3D point clouds of the study area enabled a series of measurements that were impossible to obtain in 2D images. In 3D space, the sizes (height, diameter, cup-perimeter, and cup-surface area) of several A. setubalense specimens were measured each year. The annual growth rates recorded ranged from zero (“no growth”) for a large size specimen, to an average of 2.2 cm year�����1 in cup-diameter, and 2.5 cm year�����1 in height for developing specimens. Von Bertalanffy growth parameters were estimated. Taking into account the size indicators used in this study and based on the von Bertalanffy growth model, this sponge reaches 95% maximum size at 98 years of age. During the MPA monitoring program, a high number of specimens disappeared. This raised suspicions of a phenomenon affecting the survival of this species in the area. This type of image-based methodology does not cause damage or alterations to benthic communities and should be employed in vulnerable ecosystem studies and MPA monitoring. Keywords: underwater photogrammetry, Asconema setubalense, Marine ProtectedPostprin

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

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

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

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 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 ECOMARG2017 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 (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

Analysis of the population structure of a gorgonian forest (Placogorgia sp.) using a photogrammetric 3D modeling approach at Le Danois Bank, Cantabrian Sea

The presence of gorgonian forests and deep-sea sponge aggregations in the Le Danois Bank promoted its declaration as the “El Cachucho” Marine Protected Area (MPA) by the Spanish Ministry of Environment, and its inclusion in the Natura 2000 network. Both habitats are considered vulnerable, so follow-up surveys are being performed to monitor their conservation in compliance with the EU Habitats Directive. The use of a non-invasive methodology, which does not cause damage or alterations on benthic communities, is particularly necessary in vulnerable ecosystem studies and MPA monitoring. This study analyzed the assemblage structure of a Placogorgia sp. population using a 3D photogrammetry-based method. The study was carried out through the analysis of the video transects obtained at the Le Danois Bank, using the Politolana underwater towed vehicle during the July 2017 ECOMARG survey. Recent developments in specific software of photogrammetric image analysis allowed extracting valuable information from these video transects. Using the Pix4D Mapper Pro software, 3D point clouds were obtained, and the size and morphometry of yellow fan-shaped gorgonian population structure could be evaluated. Due to gorgonian's high structural complexity, the use of length (i.e. height) as the morphometric descriptor of the real size of the colonies is not appropriate. Instead of length, the fan surface area covered by each gorgonian colony was selected as a suitable parameter of size. The direct measurement of this parameter was possible through a complete 3D reconstruction of the gorgonian forest. A total of 426 colonies of Placogorgia sp. were digitalized to obtain surface measurements and fan spread orientation calculations in 3D models. The results show that gorgonian populations were mostly composed of a high proportion of small colonies (0–0.10 m2). The population structure distribution shows a high proportion (~27%) of recruits (0.5 m2). In 78% of the gorgonian colonies, facing angles were grouped inside the first quadrant (0°-90°), in accordance with the main current direction in this zone. Colony distribution and fan orientation inside the gorgonian forest can be used as data sources to improve monitoring and management programs of these unique habitats in MPAs