Ocean acidification goes hand in hand with ocean warming and increasingly threatens life in the world’s oceans. Due to their well-developed capacities for ion- and pH-regulation, fish have generally been regarded as reasonably tolerant towards ocean acidification. Yet, there is a growing body of evidence that fish – and especially so their early life stages – also have to rearrange their aerobic metabolic pathways to meet the challenges of ocean acidification and warming, including energetic trade-offs. This puts their mitochondria into the focus of interest, which have to provide aerobic energy in sour times of increased temperatures and PCO2. I will present examples of how these challenges are met in mitochondrial acclimation of various life stages of cold temperate fish species. Maternal effects also become visible at the mitochondrial level and can additionally serve to use the full range of genotypic and phenotypic plasticity in physiological acclimation to a changing ocean.