Inter-benchmark Protocol of North Sea Whiting

The Inter-Benchmark Protocol of North Sea Whiting (IBPNSWhiting 2021) met to consider the use of updated Natural Mortality estimates from the North Sea multispecies assessment model developed by the Working Group on Multispecies Assessment Methods (WGSAM; ICES 2021a) for Whiting in Subarea 4 and Division 7.d (North Sea and eastern English Channel). In this report the estimates of Natural Mortality are compared to previous estimates (WGSAM, 2018a), the effects of this change on the assessment model are considered, and reference points recalculated. The estimates of Natural Mortality from the most recent multispecies assessment model were slightly higher than from the previous run, particularly at age 0. Incorporating these revised Natural Mortality estimates into the assessment resulted in only minor changes to the stock size, recruitment and exploitation estimates, and to the quality of the model fit. The updated model showed higher retrospective bias than previously, but was still judged to be acceptable. Follow-ing the revision of the assessment model, reference points were re-calculated following the ICES Technical guidance (ICES, 2021) using the same assumptions as for previous assessments (ICES, 2018b). This resulted in lower biomass reference point (e.g. MSY Btrigger decreased from 167 000 t to 144 000 t) and a substantial increase in FMSY (from 0.172 to 0.371).