Inter-Benchmark Process to evaluate a change in operating model for mixed fishery consid-erations in the Celtic Sea and North Sea (IBPMIXFISH).

The objective of the inter-benchmark working group was to evaluate whether FLBEIA (Garcia et al., 2017) can be used to generate advice on mixed fisheries considerations in Celtic Sea and Greater North Sea ecoregions. The working group should implement an adequate conditioning of the model and investigate the ability of the model to reproduce single species advice and pro-vide meaningful mixed fisheries advice.
The validity of the FLBEIA model implementation in both ecoregions was assessed comparing the parameterisation and forecasted indicators in mixed fisheries scenarios against those ob-tained by the stock assessment working groups and Fcube model. In both case studies the model was implemented using fleet and métier dependent catch-at-age, and discards and landings weight-at-age. In general, in both case studies, the fishing mortality at age and catch-at-age re-sulting from aggregating the fleet and métier dependent catch-at-age showed a good match with those calculated and estimated by the assessment working groups in the last assessment year. There were significant differences in some cases attributed to the procedure used to raise the data in the stock assessment working groups and the assumptions of the assessment models used.
Regarding FLBEIA model results, in Celtic Sea several problems were encountered in reproduc-ing the advice and forecast of mixed fisheries scenarios. Those were mainly attributed to the use of Cobb-Douglass model at high levels of fishing mortality and the impact of discards weight-at-age in the forecast of discards. In the North Sea the reproduction of single stock advice was adequate, and the results of the forecast of mixed fisheries scenarios obtained with FCube and FLBEIA were similar.
The group concluded that FLBEIA implementation of mixed fisheries in the Celtic Sea was not ready for provision of advice in 2021. However, for North Sea it was concluded that the FLBEIA implementation with métier dependent catch-at-age structure provides meaningful results to provide advice in mixed fisheries considerations.
The working group recommended continuing working in the Celtic Sea model conditioning, the implementation of Baranov catch production function in FLBEIA and identifying the most ap-propriate way to project over-quota discards and discard weights to be implemented later in FLBEIA. Furthermore, it also recommended to organize a dedicated workshop to address some of these issues and others identified during the meeting.

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ICES (2021). Inter-Benchmark Process to evaluate a change in operating model for mixed fishery consid-erations in the Celtic Sea and North Sea (IBPMIXFISH). ICES Scientific Reports/Rapports scientifiques du CIEM. 3 (101). 63p..,

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