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Marine ecosystems are under pressures of human activities that have altered their dynamics and structure during the last five decades, but there can also vary due to internal dynamics or chance. Managers focus on human activities and often ignore the contribution of internal dynamics and chance to ecosystem dynamics in the decision-making process. In this thesis, I use food-web modelling based on the principles of Chance and Necessity to explore the possible variability of the Nordic Seas’ food-web. I show that internal dynamics and randomness have a key role in the variability of marine food-webs. I also show that climate change and fisheries affect the variability of the Nordic Seas in a combined manner, which suggest that both stressors should not be considered separately for management. I suggest that Chance and Necessity can used a tool to inform adaptive management, ecosystem-based management, and integrated ecosystem assessment.

The ICES Working Group on comparative analyses between European Atlantic and Mediterra-nean Ecosystems to move towards an Ecosystem-based Approach to Fisheries (WGCOMEDA) recently completed its second three-year cycle. WGCOMEDA was established in 2014 and works in cooperation with other groups within the ICES Integrated Ecosystem Assessments Steering Group (IEASG).The working group objectives are: i) to set the analytical and data framework to develop common Integrated Ecosystem Assessments (IEA) exercises, ii) to advance several ongoing comparative studies on ecosystems stability, and iii) to frame comparative size- and traits-based approaches in both demersal and benthic ecosystems. To fulfil these objectives, the group focused on:1. Providing a more complete understanding of the structural and functional role of ecolog-ical stability across different types of ecosystems. The group has made considerable pro-gress on understanding the interplay between stability-diversity-resilience on both demersal and pelagic communities, conducting a resilience assessment to detect recent regime shifts, and examining the role of individual and species size in the stability of communities.2. Using functional trait information to assess the structure and functioning of demersal and benthic communities across Mediterranean and Atlantic systems and to predict their vul-nerability to fishing disturbance. Trait-based approaches (TBA) has been one of the most active topics of the group, with several Atlantic-Mediterranean comparative studies, and novel regional studies developing new approaches and methods combining trait infor-mation with methods and data beyond scientific surveys. In addition, specific trait-based approaches have been developed in the benthic realm by formulating disturbance and re-sistance indices on sensitive benthic habitats.3. Analyzing the link between ecological stability across different ecosystems types and eco-system properties (structure and function). Work on this topic has been initiated and will be one of the main objectives for the next cycle of the working group. This effort will evaluate the complexity of biodiversity patterns and investigate the link between ecosystem structure and stability.4. Identifying methods and products to support the implementation of IEA in regional eco-systems. The group discussed pros and cons of the main IEA methods and conducted an integrated trend analysis of Atlantic and Mediterranean fish communities with the goal to identify the importance of two main global drivers: fishing vs. natural hydroclimatic varia-bility. A user-friendly, open access shiny application and an R library were developed to apply this approach.WG COMEDA has collected common databases, developed novel tools, revealed common pat-terns and mechanism between the Atlantic and the Mediterranean as well as identified important differences that need to be consider in the future development of IEAs in contrasting ecosystems. Common gaps of knowledge have been also used to define new ToRs in the next WG COMEDA cycle which include: i) improving the assessment of the functional biodiversity, ii) better under-tand and integrate the complexity of marine biota, iii) investigating resilience an mechanisms of change, and iv) exploring potential options to integrate ecological and socio-economic dimen-sions.