Inter-benchmark process for witch flounder in the North Sea, Skagerrak, Kattegat, and eastern English Channel (IBPWITCH)

Type Article
Date 2021
Language English
Author(s) ICES
Contributor(s) Girardin RaphaelORCID
Source ICES Scientific Reports/Rapports scientifiques du CIEM (2618-1371) (ICES), 2021 , Vol. 3 , N. 84 , P. 105p.
DOI 10.17895/

IBPWitch was primarily tasked with establishing a new method to create reproducible survey indices after the method determined in the last full benchmark was no longer available/reproducible. In addition to establishing the new survey indices, the group also considered the assessment model configuration and updated the stock’s reference points, to ensure coherence and reproducibility across future assessments. The group selected a Generalised Additive Model modelling approach, which is implemented across many other ICES stocks, to generate indices by age and year across two quarters (Q1 and Q3). The indices’ model assumptions and configurations were thoroughly investigated, assessed and documented. The assessment model, a Statespace Assessment Model (SAM) that is available on, was modified so that age1 survey indices were not included in the model. Assumptions of interdependence in fishing pressure between ages that were adopted in the previous benchmark were tested and retained. The short-term forecast methodology was modified, to provide a more consistent estimation of recruitment. The inclusion of different data sources in the calculation of reference points was thoroughly considered and the decision made to utilise the same dataset for the stock-recruitment relationship as in the previous benchmark with just updated recent years. This relationship was modelled according to a “type-two” segmented regression and utilised to estimate Blim. Future work on this stock relies on improved age sampling (spatially and inter-lab calibration) from surveys, investigation of alternate surveys covering deeper waters, and evaluating a shift to a length-based assessment.

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