In comparison to temperate fish species, most polar fish grow more slowly and show lower relative fecundity. This might be related to specific energy requirements for cold adapted metabolic processes and low or strictly seasonal food availability. In this study we focused on the energy requiring processes.To compare the effects of temperature on metabolic energy distribution on both cellular and whole animal level, we studied high Antarctic, subantarctic and boreal fish of related species or similar ecotypes. We investigated whether the proportions of energy allocated to specific metabolic pathways in the cell underlie thermally induced changes. The energetic proportions of RNA and protein synthesis, ion regulation (Na+-K+- exchanger) and ATP synthetase were measured at temperatures ranging from 0 to 15°C. As lipids are the most important energy supply in fish the lipid class and fatty acid composition was analysed. Energy uptake and dissipation at maximum food availability as well as growth were studied at temperatures between 0 and 18 °C at the whole organism level. Results are discussed in the light of consequences for ecological preferences.

The slower growth and lower relative fecundity of Antarctic fish as compared to their temperate relatives might be related to specific energy requirements for metabolic processes in the cold and low or seasonally restricted food availability.Comparing field and experimental data, analysis of lipids and the energy budgets of the Antarctic eelpout Pachycara brachycephalum with those of the temperate species Zoarces viviparus ,energy uptake and energy allocation at different temperatures are discussed in the light of the biogeography and ecological preferences of the species.