It can easily be seen that themes discussed above may interact with each other. For example, if ocean acidification causes deteriorations of the food base of fish stocks, this can worsen the effect that any overfishing may have on these stocks. Similarly, if both acidification and bottom trawling affect benthic communities, their effects may add on or even amplify each other. A number of other interactions are possible. For example, if acidification, bottom trawling and overfishing should have a combined negative impact on food sources of seabirds and marine mammals in the future, accidental oil spills may get more serious impacts on already stressed populations of birds and mammals.
Thus, these examples clearly show that the types of themes listed in the previous section should not be considered in isolation when developing ecosystem based management. Rather, the important challenge is to conduct broad assessments of the combined impact of different types of human activities on the ecosystem. In such assessments, the influence of natural variation must also be considered.
The next question is of course how such assessments can be carried out. This is the subject of much work that is currently going into developing ecosystem based management in different countries. No attempt will be given here to review this work or give an answer to the question, but some comments will be given based on what is written in this report.
A sensible starting point is to look at the anthropogenic drivers that have the largest impact on the ecosystem, and analyse how they may interact. This is the approach taken in the chapter of this report where the overall influence of human impact on current status of ecosystem is discussed (chapter Current and expected state of the ecosystem - Conclusions about state of the ecosystem). The two drivers with the largest documented impact on the Barents Sea ecosystem is harvesting and climate change. After discussing the impact of each of them, the chapter ends with an analysis of how they may interact. In particular, it is pointed out that both temperature and fishing pressure are important determinants of stock development in Norwegian spring spawning herring and Northeast Arctic cod, and that these factors may interact through pathways that include other elements in the ecosystem, in particular capelin and zooplankton.
Another point is that assessments of combined effects may benefit from focusing on a limited set of questions. This is because the complexity may be overwhelming if all aspects of the development of the ecosystem are to be considered. Identifying relevant questions may therefore be an important step in the processes. The objectives of management will serve as useful guides in this work.
A question that will often be useful to analyse, regardless of management objectives, is whether the combined effect of anthropogenic drivers is so large that the ability of the system to absorb them is exceeded. If this happens, the ecosystem may shift to an alternative stable state, meaning that it may not shift back if the impact is reduced. This problem may be particularly important to consider when an ecosystem is under considerable pressure from several anthropogenic drivers.
In chapter Current and expected state of the ecosystem - Conclusions about state of the ecosystem this question is discussed for the Barents Sea. Comparisons are made with other ecosystems in the North Atlantic where previously dominating cod stocks have collapsed and caused the systems to shift to being dominated by arthropods and pelagic schooling fish. A conclusion is that growth in stocks of pelagic fish, such as herring, combined with a reduced ability of the cod stock to control such stocks may make the Barents Sea more prone to irreversible shifts.
This conclusion should be considered preliminary, and more research is needed to identify whether it is actually a relevant description of a critical step of the dynamics of the Barents Sea ecosystem. However, spending time and resources on such analyses may greatly improve our ability to manage biodiversity and the biological resources in the Barents Sea sustainably in a situation with considerable fishing activities and growing impact from climate change, ocean acidification and increased oil and gas activities and ship transport.
