Within the German ocean acidification research programme BIOACID (2012-2018) we investigated how the combined effects of ocean acidification and warming (OAW) affect different life stages and interactions between Atlantic cod (Gadus morhua) and Polar cod (Boreogadus saida) and their prey. Objectives included addressing the question whether OAW affects interacting species differently due to divergent physiological optima and ranges, expressed in thermal tolerance windows and associated performance capacities of specific life stages. We aimed to identify fundamental mechanisms by unravelling the connections between levels of biological organisation, from genomic, molecular to cellular, individual and population level. Scopes for acclimation (physiology and behaviour) and adaptation (evolution) that together define species resilience were studied in various life stages to identify the most sensitive one(s). Functional determinants of individual fitness such as oxygen carrying capacity, mitochondrial energy metabolism, and growth and exercise parameters, as well as their dependence on food quality and availability were also examined. In short, Polar cod displayed a higher sensitivity towards OAW than Atlantic cod that had better acclimation capacities. I will present an overview of our results, how they are implemented into future projections of a changing ocean and how these data are being used in conservation and management strategies for the two fish species.