The WKSIMULD3 meeting provided a platform for experts from the EU member states to meet and progress the assessment methodology on Criteria 3 of Descriptor 3 upon request by EC (DGENV). WKSIMULD3 is the third of a series of three workshops (WKD3C3SCOPE and WKD3C3THRESHOLDS being the first two) to identify operational indicators for MSFD D3C3.

The workshop was organised as a series of presentations with intermittent discussions. The group agreed on a number of stocks to run simulations to explore the relationships between indicators of population traits/dynamics and healthy population structure. The participants found that for demersal stocks in the North Sea, MSE simulations showed decreasing values of D3C3 indicators with increasing fishing mortality. However, the use of age-structure indicators for these stocks is likely to lead to frequent false positives due to the high interannual variability.

Age-based simulations were implemented also for pelagic and short-lived stocks, such as macke-rel, sprat, and sandeel. The distribution of the threshold values implemented for the stocks did not show a clear response to the fishing regime. As a result, the use of age-structure indicators for these stocks is likely to lead to frequent false positives/negatives. The simulations aimed to validate the population size structure and calculate indicators under different fishing scenarios.

In the Mediterranean Sea, length-based simulations were conducted for two hake stocks and one sole stock. The length indicators obtained were compared to historical data to assess the respon-siveness of the indicators to exploitation. These results show the behaviour of the length based and of the age-based indicators calculated on observed data. Overall, the indicator status can be well below the average value at Fmsy. Additionally, an exploratory run tested a harvest control rule driven by age-based indicator targets. The outputs can provide directions on how to choose a threshold that can ensure the stock to be in GES.

Overall, the workshop findings highlighted the complexity of evaluating indicators for CFP and MSFD D3 management objectives, particularly in relation to the responsiveness of the indicators to fishing pressure and environmental variation. The group emphasized the need for robust and peer-reviewed models to ensure reliable results when evaluating indicators for fish stocks. Ad-ditionally, it underscores the challenges associated with using age- and length-based indicators for different species and the importance of considering environmental and recruitment variabil-ity in simulations.

In conclusion, the series of the 3 workshops contributed valuable insights into the complexities of assessing and comparing indicators for MSFD D3 management objectives, providing im-portant considerations for future assessments and management strategies.