The boreal Atlantic cod (Gadus morhua) is entering the Arctic in response to rising water temperatures, likely increasing predation pressure on the endemic key species Polar cod (Boreogadus saida). In this thesis, I investigated the whole-animal performance of both fish species after long-term acclimation to future ocean acidification and warming conditions in order to estimate their future competitive strength. More precisely, I focused on aerobic performance such as baseline and maximum metabolism, as well as energetic investment into growth and swimming as indicators for fitness capacity under future ocean conditions. While G. morhua was thriving under conditions projected for the year 2100, the competitive strength of B. saida likely decreases. F.i., the growth performance of B. saida decreased at temperatures above 6 degree Celsius and the swimming performance was impaired under elevated PCO2 levels, potentially resulting in a higher vulnerability to predation and reduced foraging success.