As part of the European Commission's "Data Collection Framework", the MEDITS cruise is the Mediterranean component of the Sea surveys project. The geographical coverage of the program includes all trawl facades from 12 countries in the Mediterranean Sea (Spain, Italy, France, Greece, Cyprus, Malta, Slovenia, Croatia, Montenegro and Albania) and since 2009 in the Black Sea (Romania and Bulgaria). For the French part, the trawl areas are East Corsica and the Gulf of Lion. The sampling strategy is common to all countries and standardized since 1994. The standardization of the observation methods allows the reproduction of bottom trawling under similar conditions and thus the comparison of abundance indices of different species between different areas from year to year. These protocols focus mainly on the cruise period, the location of the sampling stations, the characteristics and methods of use of the capture gear, the biological observations (weighing, counting, measurements, maturity analysis, sampling of otoliths ...), as well as on data storage and sharing formats. They have been defined in common and are applied by all teams participating in the program. States must therefore ensure the collection of data and the management of these direct assessment cruises. The data from each cruise is used to provide several indicators required by fisheries managers for international expertise (GFCM -General Fisheries Commission for the Mediterranean- and STECF -Scientific, Technical and Economic Committee for Fisheries-) and national stock assessments (responses to MFAD -Maritime Fisheries and Aquaculture Directorate- referrals), communications to the profession ...) but also for the Ecosystem Approach to Fisheries, MSFD -Marine Strategy Framework Directive- (hydrology, gelatinous, marine litters, trophic chain ...) and for research purposes (projects GALION, DEMERSCAN, PhD Thesis). The data are routinely processed by the Halieutic System of Information of IFREMER to produce standardized indices of abundances. Data are also routinely shared within the international scientific community MEDITS.

For 2022, 5 additional days of survey were necessary in order to enlarge the field of observations, in particular in relation to the necessary practiced trawling, with the addition of environmental DNA (eDNA) samples by means of seawater pumping. In the Bay of Biscay, environmental DNA (eDNA) has being used for the first time for quantitative applications on hake (link between the biomass of a species and the amount of DNA it leaves in the environment). These approaches are being developed a lot for monitoring biodiversity (simple presence-absence inventory) but quantitative applications are very innovative and require significant calibration. Each species in a given region (local environmental conditions) leaves more or less genetic material behind (scales, faeces, mucus, ...) and therefore requires a specific calibration. To have the data necessary for the calibrations, from one point of view, operational pumping of seawater using the MacLane pump was carried out to collect DNA and then quantify it, in parallel with the usual trawling of MEDITS . The 5 additional days requested, made possible to carry out 11 eDNA pumping points (2.5 hours of pumping per sampling point) in the Gulf of Lion and to test the relevance of this monitoring approach when it is applied to demersal fish populations.

The development of semi-quantitative indicators is essential for establishing ecological indices and reliable biomass indicators, but to this day it remains difficult to envisage on the basis of simple approaches of environmental DNA metabarcoding.The emergence of digital PCR approaches (also called 'droplet PCR') has recently opened a way in the direction of semi-quantitative approaches, validated by recent pilot studies by obtaining conclusive calibration results in various freshwater (Thalinger et al., 2019, Hunter et al., 2018;) and coastal marine (Hamagushi et al., 2018) as well as deep (Kutti et al., 2020). This will involve participating in these emerging technological developments and helping to adapt them to the biomass estimates that are the subject of the cruises.me polymerase chain reaction (PCR) and droplet digital PCR duplex assays for detecting Zostera marina DNA in coastal sediments. Limnology and Oceanography-Methods 16:253-264.