Today's effective aquaculture production puts continually greater demands on economical and environmentally friendly production. Thus, it is essential to have basic knowledge about which factors affect the requirements of the fish for nutrients and energy and how the utilization of these dietary components is controlled. Feed utilization is influenced by a variety of factors such as the composition of the diet, the influence of the feed components on the endocrine control system, the complex interactions b etween the dietary components and environmental factors (temperature, oxygen saturation and salinity), and the psychological status of the fish. Increased knowledge about the fate of the ingested nutrients may be obtained by focusing on the digestive an d metabolic losses and the processes that control nutrient and energy metabolism. Through an integration of nutritional and physiological approaches, this program seeks to increase basic knowledge about how energy metabolism in fish