Report of the Workshop on the Analysis of the Benchmark of Cod in Subarea IV (North Sea), Division VIId (Eastern Channel) and Division IIIa (Skagerrak) (WKCOD 2011)

The ICES workshop on the analysis of the benchmark of cod in Subarea IV (North Sea), Division VIId (Eastern Channel) and Division IIIa (Skagerrak), WKCOD, met at the ICES headquarters (Copenhagen) during February 7–9, 2011. There were 19 participants from seven countries (Belgium, Denmark, Germany, Iceland, Norway, Spain, and UK), of which three participants were industry representatives and one was an external reviewer. The meeting was chaired by Einar Hjorleifsson (Iceland). The meeting was designed to serve as an inter-benchmark protocol for the North Sea cod assessment and was tasked with reviewing the input data used and the assess-ment model and settings, proposing changes to them if deemed appropriate. Additional work, if required, was also to be identified. A consensus of the data input, model assumptions and framework to be used in the 2011 cod assessment was reached at the WKCOD meeting. The conflict in the IBTS quarter 1 vs. quarter 3 surveys, an issue raised by the WGNSSK in 2010, was not fully resolved. The abundance indices in the quarter 1 survey were considered to more likely reflect stock trends in recent years, because of suspected changes in catchability in the quarter 3 survey in relation to recent changes in the spatial distribution of fish in the latter part of the year. It is recommended that further investigation be ad-dressed within a working group on improving use of survey data for assessment and advice (suggested ToRs are in WKCOD report). Additionally, using both surveys in the assessment results in trends in unallocated removals that go against the prevail-ing hypothesis that the bias in landings in particular and potentially in discards esti-mates in recent years have declined compared with the earlier period. For these reasons it was agreed to use only the quarter 1 survey in the assessment for the time being. The current assessment model (B-ADAPT) was considered to be too responsive to the noise levels in the surveys in recent years to form the basis for providing advice to a management regime which is based on the final year estimates of fishing mortality. An alternative model, SAM, which smoothes fishing mortality was adopted as the basis for an interim period (~two years) of assessments because of the more stable estimates of fishing mortality. The SAM model is considered an interim solution be-cause it estimates bias in the total catch in the same way as the previous assessment model, whereas a model that estimates bias in landings and discards separately is considered a more suitable long-term solution. Although the SAM model structure agreed at WKCOD is considered the most ap-propriate that could be fitted in the time available, a refined model structure will only be completed with further work. Consequently, WKCOD consider that if further re-finements are found to be required before the WGNSSK 2011 meeting, these be presented to that meeting for adoption (WGNSSK comprises a large part of WKCOD participants). In the medium term WKCOD considered that the development of a model structure that models discard and landings separately is required due to the differing levels of noise associated with each data set. WKCOD recommended that the reference points are not revised in the short term until the assessment model has been finalised. It was concluded that the factors for ”unallocated mortality” estimated for North Sea cod were in agreement with the general perception of the extent of underreporting except for the most recent years. However, it was also recognized that ”unallocated mortality” in general is a result of discrepancies between model assumptions and observations. Alternative model assumptions, such as changes in natural mortality or survey catchability, might also explain the patterns observed.

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