Workshop on impacts of planned changes in the North Sea IBTS (WKNSIMP)

Wieland, Kai GND; Berg, Casper; Börjesson, Patrik; Clark, Donald; Drewery, Jim; Hansson, Maria; Kempf, Alexander; Kloppmann, Matthias H. F.; Kynoch, Sven; Probst, Nikolaus GND; Sell, Anne Friederike GND; Stransky, Christoph GND; Hal, Ralf van

The Workshop on Impacts of planned changes in the North Sea International Bottom Trawl Sur-vey (WKNSIMP) objectives were to review expected short- and medium-term changes in the North Sea International Bottom Trawl Survey (NS-IBTS), evaluate the impacts of the planned changes in the NS-IBTS on data consistency for stock assessments and ecosystem indicators, and suggest strategies for the implementation of these changes with a minimum impact on future survey deliverables for data end users. An important aim of the workshop was to identify strategies how to deal with unavoidable changes such as the replacement of survey vessel and how to implement the necessary change of the survey trawl due to its divergence from the original standard and its inability to serve future needs. The agreed strategy to account for replacement of survey vessels was to ensure that there is a sufficient spatial overlap between a new vessel and existing ones operated by the other nations, and this should be done by revising rectangle allocation to the different countries where neces-sary. Several reasons discussed within the ICES International Bottom Trawl Survey Working Group (IBTSWG) have led to the decision to replace the currently used survey net GOV, most im-portantly the fact that it cannot be maintained and standardized in its currently prescribed form (additional details are provided in the IBTSWG reports of recent years). A roadmap has been developed for the implementation of the change of the survey gear. A phased introduction of the new gear in both quarters prioritising vessels that spatially overlap and for which sufficient es-timates of vessel effects are available is suggested. To maximise the ability to estimate the effect of the gear change, it is critical that other potential changes to the survey are phased in at a later date or at least not until the new survey trawl is fully implemented, in order to avoid confounding effects. Hence, the possible implementation of a survey design change from a systematic to an ecologically-based stratification has to be post-poned. Work should continue on developing and evaluating the costs and benefits of different stratification and/ or survey designs.

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Wieland, Kai / Berg, Casper / Börjesson, Patrik / et al: Workshop on impacts of planned changes in the North Sea IBTS (WKNSIMP). Copenhagen 2019. ICES.

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