Depth trajectory and performance of a trawl used for an international bottom trawl survey in the Mediterranean

An international bottom trawl survey (the Medits programme) is carried out every year in the Mediterranean since 1994. During each survey, about 1000 hauls are done by a fleet of 8 to 9 vessels from 10 to 800 metres depth, applying a unique standardized sampling protocol. This study analyses the trajectory and the performances of the sampling gear used for this survey, from records obtained during the surveys aboard some vessels in different geographical sub-areas and at different depth levels. Two kinds of sensors have been used, one to characterize a posteriori the vertical trajectory of the trawl from the beginning of shooting to the end of hauling (830 hauls), the other to describe in real time the stabilization of the trawl at its beginning (16 hauls) and its geometry during the tow (840 hauls). The results show the variability extend of these parameters linked with vessel, area, depth and period, illustrating their impact on the swept area estimates used for biological assessments. They strongly suggested the inforcement of a quality strategy combining the use of rigourous sampling protocols and systematic controling of the main parameters characterizing the trawl performance during routine surveys.