Improving the management basis for Danish data-limited fish stocks (ManDaLiS)

A substantial proportion of EU's fish stocks lack a quantitative assessment and are therefore regarded as data-limited. The goal of the present project and report, i.e. to provide and improve quantitative assessments of data-limited stocks, is therefore a prerequisite to the implementation of the Common Fisheries Policy (CFP) and to promote sustainable fisheries.

Specifically, the project and report addresses Article 2 of the CFP stating that the objective is to achieve exploitation levels that restore and maintain populations above levels which can produce the maximum sustainable yield (MSY). This cannot be achieved without quantitative assessments. Furthermore, as a result of the landing obligation (CFP article 15) data-limited stocks acting as "choke-species" in mixed fisheries have a significant risk of limiting the fishery of high-value stocks.

Obtaining quantitative assessments and fleet-specific fishing mortalities as further developed in this project is a necessary step toward mitigating the impact of choke-species and improving management of such fisheries. By improving knowledge about data-limited fish stocks, the project minimises the risk of yield reductions that result from an increased precautionary buffer applied when quantitative stock assessments are lacking.

Similarly, the risk of overexploitation is minimised with the aim to prevent a subsequent potential long-term stock rebuilding period with reduction in quotas. The specific objectives of the project is to develop and implement further newly developed statistical methods by DTU Aqua for assessment and management strategy evaluation of data limited stocks in relation to MSY (Maximum Sustainable Yield) - with focus on broad accessibility and implementation – benefitting Danish, European and global fisheries.

Furthermore, the aim is to improve the management basis for 4-6 data limited fish stocks with special importance for Danish fishery and which are managed conservatively because of limited available information. The specific stocks are selected in consultancy with stakeholders. The project has involved close collaboration with and feed-back from stakeholders within fisheries and management to ensure that the stocks selected as case studies are the most relevant to management and the fisheries industry.

The project provides quantitative knowledge and new assessments regarding more than the 4-6 Danish data-limited fish stocks that the project was originally aiming for by project start. This knowledge is not only new, but has also been necessary to enable implementation of sustainable management for the stocks.

Furthermore, the project has created the foundation for analytical assessments of these stocks and not least further development of the statistical assessment and forecast models. The sustainable management of fisheries demands the quantitative assessment of fisheries resources and exploitation patterns.

However, underlying ecological and mathematical models are often a simplified representation of natural systems and are particularly challenged if available data is limited in quantity or quality. Therefore, it is important to not only understand underlying model assumptions, but also to quantify and account for associated assessment uncertainty.

ManDaLiS has assessed several data-limited stocks with high relevance for the Danish fishery and evaluated existing and developed new data-limited assessment methods and management strategies. The focus has been on length-based assessment methods and biomass dynamic models, which are two important suits of methods for the assessment of data-limited stocks.

The performance comparison of different length-based assessment methods under various recruitment and exploitation scenarios revealed strengths and shortcomings of the various methods and provides important guidance on model selection for stock assessors and managers. A novel approach to length-based stock assessment allows the quantification of assessment uncertainty and adds a new method to the stock assessment toolbox.

The modification of the stochastic production model in continuous time (SPiCT) allows modelling time-variant productivity changes or regime shifts by a time-variant parameter for the intrinsic population growth rate. ManDaLiS has improved existing management strategies by incorporating stochastic harvest control rules which allow adjusting management advice (e.g. the total allowable catch (TAC)) as a function of assessment uncertainty.

This approach is in line with the precautionary principle and reduces the TAC if the stock status is uncertain. The methodological developments achieved within the scope of ManDaLiS shed light on the trade-offs of different data-limited assessment methods and provide new assessment methods and management strategies.

Most importantly, these developments allow the quantification and consideration of assessment uncertainty and thus, contribute to a sustainable management of fisheries. The landing obligation has drastically increased the importance of data-limited stocks as a result of the "choke-species" phenomenon.

The DTU Aqua model used (SPiCT) has been upgraded to an ICES (International Council for Exploration of the Sea) standard assessment model for data poor stocks not least due to input from the current project. Furthermore, the novelty of the model improvements provided by the project has had a high impact to both the current and future advisory and scientific assessment development work within ICES.

The case studies and model implementations have very much covered a large number of fish stocks that are important to Danish fishery. As such the models, their improvements and their implementation provide advice directly to and relevant for the Danish fisheries industry and all stakeholders within the fisheries sector.

The results and methodological developments of the project are disseminated through four scientific manuscripts, whereof one is published, one accepted, and two are pending submission for publication, and through scientific conference presentations as well as summarised in several ICES working group reports (e.g.

ICES WKLIFE VI-VIII, ICES WGNSSK, ICES WGBFAS, ICES WKMSYCat34, ICES WKPROXY, ICES WGECON). Project participants attended these working groups to make specific recommendations regarding future data calls, methodological directions, and assessment, advice and management strategies in general and for several stocks including stocks where a robust assessment cannot be provided.

Furthermore, the implementation of the models have been affiliated further through ManDaLiS contributions to other EU projects covering the EU-Tender DRUMFISH, EU-Tender PROBYFISH, EU Tender EFICA, and EU-H2020-MEESO, and not least conducting a full PhD Study cofinanced between ManDaLiS (2 years) and a DTU Aqua internal PhD project (1 year) with input to stock assessments, methodological reviews and improved assessment methods.

As such, the ManDaLiS project has also been further implemented and disseminated through the cooperation in international expert networks working under these international research projects, as well as implementation of the model developments under ManDaLiS in the assessments conducted under those research projects.

There has been conducted three project workshops held in cooperation between the EMFF ManDaLiS and MSPTOOLS projects. One of the workshops was international and was held in association with and just after an International Conference Special Session: IIFET Conference, Seattle, USA, July 2018, (IIFET 2018 International Institute of Fisheries Economics and Trade, https://www.xcdsystem.com/iifet/website/).

This Special Open Session was directly arranged by the ManDaLiS and MSPTOOLS projects with invitation of stakeholders and including stakeholder perspectives. Besides initiative taking, planning, arranging, organizing, coordinating, announcing, leading and carrying through this special session directly under the MSPTOOLS and ManDaLiS Projects the projects produced the session abstract and a full scientific publication reporting of the outcomes of the session (Nielsen et al., 2018).

In accordance with several of the stakeholder perspectives from the above workshops and the IIFET session future perspectives and needs raised by the stakeholders during the ManDaLiS and MSPTOOLS workshops included suggestions for continuation of the implementation of the data poor stock assessments and model developments on additional candidate stocks with importance for Danish fishery.