Benthic invertebrates’ by-catches in Ifremer bottom trawl surveys in the English Channel and Southern North Sea: 2006-2018 observations

Bottom trawling mostly targets fish and cephalopods while mega-zooplancton and most other benthic invertebrates are considered as by-catches of this fishing technique. The mesh size of bottom trawl, generally decreasing from opening to cod-end, and the ground gear used (usually only scratching the seabed surface) result in mostly mega-(>2cm) epifauna being captured. Although bottom trawl is seldom recognised as a valid sampling device for benthic invertebrate species, such observations are nonetheless believed to be particularly relevant as 1) they represent the benthic fauna fraction the most likely affected by bottom fishing 2) they integrate assemblages’ composition over large areas (3-4 km long and 10-20 m large) and are more representative of larger scale habitat structure and 3) they are particularly suited to collect over-dispersed or motile species. IFREMER contributes to the collection of basic biological data in the English Channel and North Sea through its annual bottom trawl surveys, the CGFS (Channel Ground Fish Survey, carried out in October since 1988 on board of the RVs Gwen Drez and later Thalassa, Coppin and Travers-Trolet, 1989) and the IBTS (International Bottom Trawl Survey, undertaken in January/February since 1970 on board of the RV Thalassa, Verin, 1992). Since 2006, all megabenthic invertebrates captured in the trawl have been identified, counted and weighted. Additionally, in September 2014, IFREMER carried out a bottom trawl scientific survey, CAMANOC (Campagne Manche Occidentale, Travers-Trolet and Verin, 2014), on board of the RV Thalassa in the western English Channel, where megabenthic invertebrates caught in the trawl were also identified. The CGFS had a fixed sampling design while IBTS and CAMANOC had a random stratified sampling strategy but with varying intensity depending on the covered survey area. Their data merged together cover the entire English Channel and southern part of the North Sea although there are much more observations and longer time series in the eastern part of the Channel than in the western part. For all three surveys, the sampling gears used were all Very High Vertical Opening (VHVO) bottom trawls (or “GOV”), well adapted for catching demersal species (in particular fish and cephalopods), with a 10 mm mesh size at the cod-end for catching juveniles. The sampling strategy was using standard 30 minutes hauls at 4 knot speed during daylight, evenly distributed over the whole study area. Demersal species and megafauna/epifauna caught in the bottom trawl were sorted, identified, counted and weighed (ICES, 2017). The present dataset focuses on benthic invertebrates and is composed of two tables. The haul table gives information about each operation (survey, date, location, gear type, swept area in km²). The catch table gives information on species catch raised as total number or total weight (g) in the haul. It is strongly recommended to standardise these values per km² as the swept area may vary vastly from one observation to the next, due to both current speed and difference in gear size. When abundance or weight were not evaluated, the value “-1” is used and marks the presence of the species in the catch. Colonial species are not generally counted.