The Marine Strategy Framework Directive (MSFD) complements European fish stock assessments under the Common Fisheries Policy (CFP). CFP assessments separately assess fishing mortality (F) and spawning stock biomass (SSB). Contrary, within its third descriptor (D3) of good environmental status (GES) the MSFD requires to assess exploited fish stocks against three criteria, which are F (D3C1), SSB (D3C2) and the age or size structure (D3C3) within the stock. Further, the MSFD requires to integrate the status of all three criteria to determine whether a stock has achieved GES. The full implementation of MSFD compliant stock assessments has been impaired by the lack of operational indicators for age/size structure. This study presents an approach to assess D3C3 by analysing two indicators i.e. recruitment (R) and SSB/R using time series-based assessments. R and SSB/ R reflect the small and large components of a stock and are used as proxy indicators for stock productivity and realised growth potential. Using stock assessment data from 20 North East Atlantic fish stocks, a RandomForest model validated the sensitivity of SSB/R against F and two selectivity indicators. Further, this study introduces an approach to integrate the assessment outcomes of the three D3-criteria by stock. The here demonstrated approach of assessing fish stocks according to D3 of the MSFD relies only on data available from stock assessment summary sheets and is thereby easy to implement for all stocks with analytical stock assessments. The impli-cations of and future direction for assessing D3C3 are discussed.
