Arctic marine organisms experience high variation in food availability over an annual cycle, causing strong seasonal fluctuations in the energy metabolism of an organism. Energy homeostasis is, however, a prerequisite to perform biological processes such as reproductive development, growth and allocate energy to endogenous defense mechanisms in response to pollution events. Increased industrial activities related to oil production and transport are projected for the Northern Barents Sea in near future, which implicate the risk for oil pollution in Arctic waters. The present study aims to examine the sensitivity of polar cod (Boreogadus saida), a key species of the Arctic marine ecosystem, to petroleum compounds during polar night. Polar cod spawns in early winter and utilizes the majority of its energy reserves for reproduction, which is partially restored through food intake in early spring. We hypothesize that low energy reserves related to restricted food access and reproduction render polar cod more sensitive to petroleum exposure and alter post-spawning recovery. Polar cod will be exposed to the water-soluble fraction of crude oil in an experiment that simulate a winter oil spill scenario and will also be subjected to different food regimes from a late gonadal maturation stage (December) until post-spawning (March). Fish sensitivity will be examined with biomarkers related to energy allocation, fecundity and growth. Additionally, ultrasonography will be used to determine the sex of polar cod prior to experiment start, a new method to decrease inter-specific variability in sex ratio between treatment groups and thereby increase the significance of experimental results.