The end-to-end model Atlantis has been set up for the Nordic and Barents Seas, covering an area of totally 4 million km^2. The model has been under extensive tuning, as it is the first time it has been adapted to such high latitudes. To represent the ecosystem in the area, 52 functional groups and species have been parameterized, covering all trophic levels from bacteria, phytoplankton, zooplankton and fish to marine mammals. Through the tuning process, there has been made a lot of “guesstimates” and approximations. We will present the vulnerability of the model to some of these, such as mortality rates, feeding rates and growth rates. The difference between making guesstimates on growth and mortality rates in comparison with feeding interactions will be presented for a few chosen species. In addition to direct effects, the indirect effects due to feeding interactions will be explored. Through the study we will identify important areas where more information and research is needed, with focus on both commercially important species, but also on vulnerable species.