Working Group on Baltic International Fish Survey (WGBIFS; outputs from 2022 meeting)

The Baltic International Fish Survey Working Group (WGBIFS) plans, coordinates, and implements demersal trawl surveys and hydroacoustic surveys in the Baltic Sea including the Baltic International Acoustic Survey (BIAS), the Baltic Acoustic Spring Survey (BASS), and the Baltic International Trawl Surveys (BITS) in the 1st and 4th quarter on an annual basis. The group compiles results from these surveys and provides the herring, sprat, cod and flatfish abundance indices for the Baltic Fisheries Assessment Working Group (WGBFAS) to use as tuning fleets.

In 2022, WGBIFS completed the following tasks: (1) compiled survey results from 2021 and the first half of 2022, (2) planned and coordinated all Baltic fish stocks assessment relevant surveys for the second half of 2022 and the first half of 2023, (3) updated the common survey manuals according to decisions made during the annual WGBIFS meeting. Data from the recent BITS was added to the ICES Database of Trawl Surveys (DATRAS). The Tow-Database, which allows planning the spatial distribution of hauls in the areas where the seabed is suitable for safety trawling, was corrected and updated. The Access-databases for aggregated acoustic data and the ICES database of acoustic-trawl surveys for disaggregated data were also updated. All countries also registered collected litter materials to DATRAS.

The area coverage and the number of control hauls in the BASS and in the BIAS in 2021 were considered to be appropriate for the calculation of tuning indices and the data can be used for the assessment of Baltic herring and sprat stocks. The number of valid hauls accomplished during the 4th quarter 2021 and 1st quarter 2022 BITS were considered by the group as appropriate to tuning series and the data can be used for the assessment of Baltic and Kattegat cod and flatfish stocks. BIAS and BASS survey sampling variance calculation questions were discussed and standard deviation for Central Baltic herring acoustic index series calculated.

A preliminary comparison exercise between the StoX survey computational method and traditional IBAS calculation methods was performed for the Central Baltic herring and Baltic sprat. It was found that the StoX project, developed for the WGBIFS, has small methodological differences compared to the standard calculation method used by the group, as specified in the Manual for the International Baltic Acoustic Surveys, (IBAS) and is thereby causing a small difference in the total number of herring and sprat. Larger differences appear in abundance estimates at the ICES Subdivision and rectangle level and in the age composition as well. These differences together with errors found in the database will be mapped before the next meeting and the work towards a possible transitioning to StoX software will continue.

A further comparison exercise between the StoX method and the current Gulf of Riga Herring Survey calculation method was performed using data from 11 last years. It showed no major differences in herring total abundance estimates for most of the years. However, notable differences were in the age compositions of those two methods. Some errors and differences in input data (uploaded into the ICES database) were found and therefore the further analysis was postponed until these issues are fixed.

WGBIFS is planning to continue with analogical comparison exercises in the coming years before the final transition to StoX and to a transparent reproducible pathway into the ICES Transparent Assessment Framework (TAF) can be done. Work towards transitioning to TAF will continue before and during the next meeting until all methodological and database differences are resolved.

Inquiries from other ICES expert groups were discussed and addressed.



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