Four demersal stocks were Benchmarked in WKDEM, Haddock (Melanogrammus aeglefinus) in subareas 1 and 2 (Northeast Arctic) (Had.27.1–2), Cod (Gadus morhua) in Division 6.a (West of Scotland) (Cod.27.6a), Whiting (Merlangius merlangus) in Division 6.a (West of Scotland) (Whg.27.6a) and Whiting in Division 3.a (Skagerrak and Kattegat) (Whg.27.3a).
The 2020 benchmark for Northeast Arctic Haddock was proposed due to the poor retrospective pattern (annually changing perception of the stock) for the assessment in 2018. The final SAM model was based on the updated dataset and a new SAM setting allowing for plus groups in surveys. The final model had a lower AIC and more parameters compared to the configuration used in the previous assessment with the same data as the final model. Some changes were made to the prediction models, mainly to reflect the current perception of the stock dynamics. The reference points were not changed.
The 2020 benchmark for 6.a cod was proposed initially due to uncertainties in the catch data and the selectivity patterns in the catch and survey data. In addition, a separate analysis of the stock estimated a different trend in fishing mortality and higher biomass, which lead to an inter-benchmark in February 2019. A number of alternative assessment methods were considered with a decision to switch from the TSA model to SAM. An improved approach to estimating area-misreported landings was implemented, and maturity and natural mortality values were modified. The survey data included in the final assessment are consistent (in terms of year and age range) with those used previously. The reference points were updated having been derived following the ICES guidelines and using EqSim. There was some concern that an alternative model, FAF
model, along with an extension to allow natural mortality to be estimated by the model (“TVM” model) provided a different perception of the stock dynamics. The latter model was not presented for evaluation prior to the Benchmark and as such needs to be evaluated as to its efficacy for assessment of this stock in the future.
The 2020 benchmark for 6.a whiting was proposed because the assessment was influenced partly by incorrect reporting of landings data (species and quantity) from the past which directly affected the perception of the stock. There were newly available catch data and reworked survey indices (including the combined Irish and Scottish Q4 survey), plus reworked maturity and natural mortality estimates were available. It became clear that running TSA with the new data and changed survey configuration resulted in poorly converged optimisation runs. An age-aggregated stochastic Surplus Production in Continuous Time (SPiCT) model was tried but the modelled stock level was highly uncertain, although the trends were considered more robust. The result was to reclassify 6.a whiting to category 3 using the SPiCT model to give trends, according to the ICES guidelines for data-limited stocks.
The 2020 benchmark for 3.a whiting was proposed because it was considered a category 5 stock with no advice and more information is available (time-series of catches (landings and discards), a new fisheries independent biomass index that combines all relevant scientific surveys in the area) that were not being used but could be utilised to infer stock status and provide better catch advice. The outcome was to reclassify the stock to category 3 and provide advice using the “2-over-3” trend based approach with the new biomass index. An attempt to provide an analytical assessment for the stock using the surplus production model (SPiCT) was unsuccessful.