Reza it sounds to me like you are really asking two different questions. The primary question about measuring energy content in body with protein and lipid content can be addressed. The sub question of minimum food intake to sustain metabolism and drive muscles would require additional measures of body composition over time to see how much energy was depleted over time. Lots of things will affect this energy use like temperatures experienced, body size, fasting period, activity, etc. Assuming that you want to do this without sacrificing the animal there are several ways to estimate the body composition without the need to kill and analyze the animal. In fish, there are microwave technologies as well as bioelectrical impedance analysis (BIA) methods. I can provide some references for both of these methods upon request. Using BIA (which I am familiar with) you can estimate the water content and from that the protein and fat contents are easily and reliably estimated. Using mass of fat and protein and published values of the energy content of each you should be able to estimate the energy used over time under the conditions the fish experiences.