First approach to study the size and morphology of gorgonian corals and sponges using photogrammetric analysis from video transects in the Marine Protected Area ‘El Cachucho’ (Cantabrian Sea)

Abstract

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. The European directives on protection and the OSPAR Commission described the habitats structured by sessile species of great size, such as corals or sponges, as particularly vulnerable habitats requiring special protection. The knowledge of population structures as indicators of their state of health is a requirement to improve the management programs. The presence of gorgonian forests and deep-sea sponge aggregations in the Le Danois Bank was the cause of it declaration by the Ministry of the Environment as "El Cachucho” MPA, being included in the Natura 2000 network. Currently being performed follow-up surveys to know their conservation status.

This study presents methodology and first results obtained in the analysis of the video transects acquired in ‘El Cachucho’ MPA, during the ESMAREC survey using the ROTV Politolana underwater towed vehicle. Video was recorded to pilot the ROTV, so characteristics of video camera were not specifically designed for monitoring the habitats. But recent developments in specific software of photogrammetric image analysis have allowed extracting valuable information of these video transects. The outputs of the photogrammetric approach include products such as geo-referenced orthomosaic, Digital Surface Model (DSM) and point cloud (XYZ).

These data show the possibility to evaluate the size and morphology (including 3D analysis) of the vulnerable species in the area, as the gorgonians Paramuricea placomus and Callogorgia verticillata and the sponges Asconema setubalense and Geodia megastrella. The non-invasive growth rate studies need a set of measurement, fan height, fan width, and others. This methodology will make easier future studies about age and rate of growth of these species and population structure in the zone.