The International Bottom Trawl Survey Working Group (IBTSWG) coordinates fishery-inde-pendent multispecies bottom-trawl surveys within the ICES area. These long term monitoring surveys of demersal fish provide data on commercial species for stock assessments and facilitate examination of changes in fish distribution and abundance. The group also promotes the stand-ardization of fishing gears and methods and survey coordination.
This report summarizes the national contributions in 2019–2020 and plans for the 2020–2021 sur-veys coordinated by IBTSWG. In the North Sea, the surveys are performed in quarters (Q) Q1 and Q3 while in the Northeast Atlantic the surveys are conducted in Q1, Q3, and Q4 with a suite of 14 national surveys covering a large area of continental shelf that ranges from North of Scot-land to the Gulf of Cádiz.
The 2019-2020 sampling plan was generally completed for all areas. Some deviations concern the Portuguese survey (PT-PGFS-Q4), which was cancelled for reasons that currently remain unre-solved for the 2020 survey. The Channel Groundfish Survey (CGFS) was again extended into the western channel. The French Evaluation Halieutique Ouest De L'Europe (FR-EVHOE-Q4) sur-vey was affected by weather, but the Irish vessel was able to survey most of the missed stations in the Celtic Sea strata. The North Sea International Bottom Trawl Survey (NS-IBTS Q3) found high densities of cod, Norway pout and mackerel outside the index areas. The NS-IBTS Q1 in 2020 was affected by Germany not receiving a permit to fish in UK-waters. This was addressed by redistribution of many stations to other countries. The result is a different distribution of sta-tions by country, and a reduced amount of overlap between countries to estimate the coun-try/vessel-effect. Furthermore, the survey was hampered by two storms resulting in a lower number of GOV and MIK-hauls. This survey observed a very large number of 1-ringer haddock, even as far south as the southern Dutch coast.
Evaluation of methods resulted in corrections for some parameters used to calculate swept-area in the Database of Trawl Surveys (DATRAS) and algorithms were provided to fill in missing values for the NE-Atlantic surveys.
The Scottish organized trials to test new gears proposed for use by the Scottish and Irish. Trials were conducted on board R/V Scotia with participation by both countries and the Dutch. The trials showed more stable gear performance than for the Grande Ouverture Verticale (GOV) trawl and catch data indicated that all species were caught. However, there were differences in catch composition, with the Scottish gear capturing more pelagics (higher net opening), while the Irish gear included more flatfish and other fish species near the bottom (difference in groundrope). These differences may be resolved by adjusting the net opening and groundrope to make the nets more similar. Dutch observers confirmed that both nets operated properly and could be used on their vessels without adjustments. However, they preferred that the Irish gear be made with heavier/stronger netting material and the Scottish gear to include another groundrope. IBTSWG will continue evaluation of these new gears.