Climate change is causing rapid warming of Arctic regions threatening the ecosystem and their species such as the polar cod (Boreogadus saida). Polar cod is considered the most abundant forage fish species linking lower and higher trophic levels in the Arctic food web. In particular, the thermal tolerance of sensitive life stages, such as reproducing adults, is important for an assessment of biodiversity changes in the context of global warming. Here, a non-invasive experimental approach using magnetic resonance imaging (MRI) is presented to investigate the physiological performance of reproducing male, female and non-reproducing polar cod during acute warming. Inside the MRI scanner, 0°C pre-acclimated polar cod were exposed to a temperature ramp from 0°C to 8.5°C at a rate of 1.5 °C every two hours. A set of physiological performance parameters of the cardio-vascular system and energy status were continuously determined during the temperature ramp. In particular, flow-weighted MRI techniques were used to quantify blood flow in the head and in the abdomen, respectively, and the ventilation rate was assessed using CINE MRI. The energy status was determined by in vivo 31P-NMR spectroscopy. A preliminary data analysis showed a similar blood flow at the control temperature of 0°C for all three animal groups, whereas blood flow increased remarkably in the head of reproducing males during warming, which was more pronounced in comparison to non-reproducing individuals. These initial results could indicate a higher energy requirement of the reproducing males, leading to a higher sensitivity to future warming conditions.