International Bottom Trawl Survey Working Group (IBTSWG)
This report summarises the national contributions in 2018–2019 and the planning for the 2019– 2020 surveys coordinated by the International Bottom Trawl Survey Working Group (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 the continental shelf that ranges from North of Scotland to the Gulf of Cádiz.
The sampling plan was generally completed for all areas. Some deviations concern the Portuguese survey (PT-PGFS-Q4); 12 days were lost due to weather conditions (around 60% of the stations completed). Twelve additional hauls were undertaken aboard the commercial vessel Calypso. The Channel Groundfish Survey (CGFS) extended the area into the western channel. EVHOE survey started to collect juvenile mackerel. Among specific results, the abundance of large herring larvae was very low in the eastern North Sea (Q1, 2019) and virtually no larvae occurred in the German Bight. From the MIK samples above-average catches of pilchard larvae were found specifically in the German Bight. Pilchard in the GOV catches was also above average. High densities of some target species were found outside the actual index areas during the North Sea Q3 (2018, e.g. Norway pout). Actual distribution patterns may warrant a revision of the species-specific areas on which the standard indices are calculated in DATRAS.
About DATRAS related topics IBTSWG adopted the conclusion that swept-area based survey indices require tow by tow information to be included into the database and the upload of new algorithms for the calculation of the swept area data products will be allowed. Tests for the new trawl gear undertaken by the Irish and the Scottish participants are in progress. The first results of sea trials of the two design approaches carried out in 2018 are detailed in this report.
Analysis of the impact of changes in the design of the survey on the advisory processes is still in its early development and focused on three aspects: implementing different ways of sub-sampling, implementing different methods of index calculation (model and design based indices) and automating the assessment process in an effective way to enable the evaluation of large numbers of simulations. Moreover, an evaluation of sampling strategies of otoliths focused on three objectives: development of spatial age-length key (ALK) for estimating indices of abundance at age, development of uncertainty estimators for abundance at age indices and ALK estimators and investigation of the effect of the number of otoliths sampled or hauls on abundance at age indices.
About survey design and standardization process, the efforts and methods of standardization of the combined US and Canadian surveys (Northeast Fisheries Science Center) have been presented. Finally, more general topics and scientific results were presented: example of the evolution towards an "ecosystemic survey" from EVHOE survey; results on the reorganization (‘tropicalization’) of the ichthyological community structure in the North Sea.
Published under the auspices of the following ICES Steering Group or Committee