Benchmark Workshop on Selected Plaice Stocks (WKBPLAICE)

The Benchmark Workshop on selected plaice stocks (WKBPLAICE) evaluated the assessment (input data and methodology), short–term forecast procedures and reference points for three plaice stocks, plaice (Pleuronectes platessa) in Division 7.e (western English Channel), plaice (Pleuronectes platessa) in subdivisions 27.21-23 (Kattegat, Belt Seas, and the Sound) and plaice (Pleuronectes platessa) in subdivisions 27.24-32 (Baltic Sea, excluding the Sound and Belt Seas). The key focus has been on evaluating the stock structure of the Baltic Sea plaice and conducting a stock-specific Management Strategy Evaluation (MSE) to tune a Category 3 empirical harvest control rule for plaice in Division 7.e. Plaice in Division 7.e is a category 3 data-limited stock applying a chr-rule, which is defined by optimizing the Itrigger and harvest rate. The two Baltic plaice stocks were merged during the benchmark process resulting in one stock, plaice in subdivisions 27.21-32. The stock was classified as a category 1 stock which is assessed with State-space assessment Model (SAM).

Plaice (Pleuronectes platessa) in Division 7.e (western English Channel)

The previous stock assessment for plaice in Division 7.e was classified as a category 3 stock and advice was given with the rfb rule. Due to large uncertainties in data and assumptions, the model was not moved into category 1. Instead, a stock-specific simulation (management strategy evaluation, MSE) was conducted to tune the chr rule, an empirical harvest control rule based on the relative harvest rate. Various input data were reviewed and updated, with discard survival assumed to be 50%, though different scenarios were considered. A total of 14 age-structured stochastic operating models were developed, and the chr rule was tuned using a set of seven reference models to achieve both Maximum Sustainable Yield (MSY) and a precautionary approach (keeping the stock above a critical threshold). The final tuned chr rule (MP5) provides biennial advice, uses the UK-FSP survey as a biomass index, and was found to produce high long-term catch and stock biomass. Robustness tests showed superior performance to other MPs across all scenarios. When compared to the previous rfb rule and the standard ICES MSY rule, MP5 yielded a higher catch, while the ICES MSY rule did not fully meet precautionary approach standards. No recommendations for further investigations are outlined in the reviewers’ report and the work was accepted by the whole group.

Plaice (Pleuronectes platessa) in Subdivision 21-32 (Baltic Sea)

The stock structure of plaice in the Baltic Sea was extensively reviewed. WKBPLAICE made the decision to merge the two Baltic Sea plaice stocks into one single stock, plaice in subdivisions 27.21-32 (Baltic Sea).

Plaice in the Baltic Sea is now assessed as a category 1 single stock using the SAM. Various input data were reviewed and updated after group discussions. Survey data coming from two bottom trawl surveys are combined using Delta-GAM models to produce a stock index. Discard survivability was considered, with three scenarios tested (low, medium, high), which affected the spawning stock biomass (SSB) estimates but had less impact on fishing mortality. After a group discussion, the medium discard survival scenario was chosen. Natural mortality scenarios were also evaluated, included age- and time-variant natural mortality, and various SAM configurations. Proposed models were evaluated following the ICES guidelines for benchmarks. Following extensive discussions, the final assessment, reference points and short-term forecasts following ICES guidelines were agreed by the group. As not all points from the issue list could be handled during the benchmark, recommendations for further investigations are outlined in the reviewer’s report and issue list. The stock annex will be prepared in advance to WGBFAS.

General findings

The group collectively approved of the assessment frameworks for both stocks proposed after several revisions and modifications throughout the benchmark process. For plaice in division 7.e, the approach of defining the chr rule based on an optimization of the chr parameters is seldom done, but as shown here resulted in superior outcomes in the long-term according to the chr rule chosen. In this case, the best I_trigger chr parameter value was shown to occur at relatively high value, which led to better stock outcomes due to a faster recovery rate when the stock falls to low levels. This may be an interesting property to explore in the future for generality. For plaice in subdivisions 27.21-32, the stock assessment itself was found to be informative, but there was rather high uncertainty in assumptions made regarding future productivity. This results in reference points that should be taken with caution, but it is also not an uncommon situation in a continuously changing environment and still adheres to ICES guidelines. In the future it may be useful to approach the reference points using an ensemble approach to incorporate more uncertainty, and this approach could be encouraged within ICES frameworks.