This report describes the process undertaken by ICES to provide guidance to support EU Member States (MS) in the implementation of the Marine Strategy Framework Directive(MSFD) Descriptor 3 (D3), commercially exploited fish and shellfish. The report also describes the potential role of ‘ecosystem’ indicators collected under the DCF to support assessments of other MSFD Descriptors. Five main steps were identified to assess Good Environmental Status GES for D3: • Selection of commercially exploited (shell)fish populations relevant to the MSFD (sub)region, or MS-specific sub-division, being assessed with respect to D3; • Identification of stocks that can be assessed in relation to the primary assessment criteria for D3.1 and D3.2; • Determination of criteria to apply to stocks that can not be assessed in relation to the primary assessment criteria, and identification of stocks that can be assessed according to these secondary criteria; • Interpretation of how to define GES for D3 with respect to combining individual stock assessments at the criteria level, and how to combine criteria level assessments at the descriptor level; • Assessment of current status in relation to GES. Different approaches for conducting these five steps towards assessment were applied in 5 case studies covering most of the MSFD (sub)regions, i.e. the Baltic Sea, Mediterranean Sea, North-east Atlantic Ocean – Bay of Biscay and Iberian Coast, North-east Atlantic Ocean – North Sea and North-east Atlantic Ocean – Celtic Seas. For the selection of what can be considered the commercially exploited (shell)fish in a particular (sub)region, the following key issues were identified: (1) Identification of the appropriate area; (2) Match of existing spatial units to that area; (3) Choice of data source; (4) Choice of time period; (5) Selection criteria. While each of these issues was seen to have some consequences for the selection of relevant populations, the overall assessment appeared airly robust against a range of sensible choices. For commercially exploited (shell)fish populations with assessments, primary indicators and MSY-based and/or precautionary reference levels are defined. As the assessed stocks do not always match the MS’s marine waters, issues pertaining to the selection of stocks considered representative for the MS’s waters arise. Another issue in the selection of assessed stocks to be examined under D3 oncerned the quality of the assessments and, thus, the information they provide, i.e. (1) all indicators with reference levels, (2) not all reference levels, or (3) no reference levels. As the assessed stocks can be considered the best source of information, any decision on these aspects may have significant consequences for the GES assessment. For commercial populations that do not have full assessments scientific monitoring surveys were identified as a potential data source for calculating some secondary indicators. Three options for determining the current status from trend-based timeseries were considered: (1) comparing the recent period mean with the long-term average (2) comparing the current value of the indicator in relation to the historic mean setting a threshold based on appropriate percentile of the Normal distribution; (3) detection of trends. However it is noted that trends based methods do not provide specific definition of reference levels in relation to ‘good’ status, and can only provide an indication of change. None of the considered methods were evaluated, and therefore no recommendations are provided with regards to secondary indicators for criteria 3.1 and 3.2 or criterion 3.3. It was noted that the ‘mean maximum length across all species’ indicator proposed under criterion 3.3 is not appropriate as a stock condition metric and it is not advised for application under Descriptor 3. An analysis comparing the outcomes of the GES assessments based on indicators with (from stock assessments) and without reference levels (from monitoring programs) showed some consistency, but also revealed that the GES assessment based on indicators with reference values is more strict than the one based on indicators without them. This is because with a relatively short time series the historic mean may still be far from where GES would ctually be (and which should be represented by the MSY-based reference levels). Three possible definitions of GES at the criterion level were considered reflecting different levels of ambition: • GES Interpretation 1: strict interpretation of the Commission Decision where MSY reference levels are treated as a limit and thus all stocks must meet the MSY requirement • GES Interpretation 2: the MSY reference levels are considered as a target and thus half the stocks must achieve the MSY requirement, and all stocks must achieve precautionary reference levels • GES Interpretation 3: the MSY reference levels are considered as a target and stocks need to achieve this requirement on average. This average is calculated accounting for the ‘distance’ individual stocks are above or below the MSY reference level. The examples provided in the report confirmed that the interpretation of GES can have important consequences for the outcome of the GES assessment. A set of rules is provided that shows different ways that criteria may be combined (or not) for an overall assessment of current status in relation to GES. For the overall assessment of Descriptor 3, three approaches were considered in the case studies: (1) no aggregation across criteria; (2) application of the one-out-all-out aggregation rule or “assessment by worst case”; or (3) application of weights for the different criteria. Evaluation of the quality of the GES assessment should be provided. The quality of the assessment depends on the proportion of species/taxa that have information according to certain quality standards. A higher proportion of assessed stocks increases the quality of the GES assessment. Similarly, a higher proportion of species/taxa for which no information is available decreases the quality. The quality also increases with increasing length of the time-series of indicators without reference levels, to the extent that sufficiently long time-series would result in an assessment that could perform as well as one based on indicators with reference values. What can be considered “acceptable quality” remains unresolved but the different case studies explored a range of varying quality. Finally some fisheries related indicators used by various organizations (i.e. EEA, Eurostat) were assessed with a view of simplifying/reducing the number of indicators and at the same time using the DCF data. Based on the assessment a potential framework for a core set of ICES indicators on ecological impacts of fishing was proposed. The aim is that ICES will calculate and publish these annually as part of the planned ecosystem overviews. For the DCF indicators (Conservation status of fish species, Proportion of large fish, Mean maximum length of fish, Areas not impacted by mobile bottom gears) the availability of reference levels was assessed and comments provided on how they could be applied to support MSFD assessments. For some of the indicators the need for modifications or further development of the indicators and their calculation was suggested, and some modifications were proposed.