Workshop on Stock Identification of West of Scotland Sea Cod (WK6aCodID; outputs from 2021 meeting)

The Workshop on stock identification of West of Scotland cod (WK6aCodID) convened to recommend the most plausible scenario of population structure for stock assessment and fishery management advice. The review considered geographic variation and movements of cod lifestages inferred from genetic analyses, scientific surveys, fishery data, tagging, and otolith microchemistry and shape. Based on the review, several population structure scenarios were hypothesized (including the scenario assumed in the current advisory unit), and the plausibility of each scenario was evaluated. Practical implications of the most plausible scenario, including the derivation of a catch time-series, were considered to form recommendations for benchmark stock assessment workshops.

The workshop considered three hypotheses - hypothesis 1: closed homogenous population; hypothesis 2: a metapopulation with overlapping subpopulations (but not necessarily with areas within Division 6.a) (Clyde, Dogger inshore, Dogger offshore) and hypothesis 3: multiple overlapping subpopulations related to Dogger stocks (between Division 4.a and Division 6.a) and a separate subpopulation of Clyde.

While hypothesis 3 provides the strongest scientific evidence, treating the Clyde as a separate population is problematic with respect to reconstructing historical catches and undertaking assessments. Over the last two decades of local management with the Clyde cod closure, incorporating minimal catches into subpopulations is unlikely to impact assessments at the present time.

Given the current weak state of Clyde cod, hypotheses 2 and 3 would be practically indistinguishable in terms of assessment outcome and the workshop considers hypothesis 2 amenable to stock assessment in the short- to medium-term, while investigations continue into the assessment of the Clyde cod and the improvement of data availability. Given linkages of the inshore and offshore subpopulations to cod in Division 4.a, it is recommended to combine the North Sea and West of Scotland cod assessments in a future benchmark.

The workshop elaborated two scenarios with respect to a potential ICES data submission consistent with the modelling approaches being considered for the North Sea cod stock (cod in Subarea 4, Division 7.d and Subdivision 20); namely, either that data are submitted for the whole of Division 6.a as for the current ICES stock assessment, or that data are split into stock components as defined under hypothesis 2 (and hypothesis 3). In these cases, the workshop suggested that two approaches be considered based on data availability and a conversation between ICES national data submitters and ICES stock assessors.

ICES data submitters and ICES stock assessors should agree on a suitable time period for data splitting; considering the current assessment time-series for both West of Scotland and North Sea stocks and data quality back in time.

Having addressed its Terms of Reference, the workshop noted that there is potential for much reshaping of Atlantic cod stock assessments currently. With the four primary cod assessments (Celtic Sea, Irish Sea, West of Scotland and North Sea) presently exhibiting issues. Primarily these revolve around continued low catch tonnage, which translates into low catch numbers-at-age and resolves with heightened uncertainties in assessments. To investigate and review further, the workshop proposes that a planned approach be developed within ICES, through initial roadmapping for improving assessments and the basis for advice.