Total 150 samples of complete wild freshwater yellow eel (7.5 – 46.9 cm; 0.42 – 174.3 g wet wt.) were analyzed for percentage water and content of ash and Kcal (i.e. energy) from bomb calorimetry. Content of protein per g dry wt. was calculated from values of Kcal, and content of lipid per g dry wt. was calculated as a difference between contents of protein and ash. Percentage water was highly negative correlated with eel body mass g wet wt. Lipid content per g body wet wt. increased highly with eel body mass. This was in contrast to the constant contents of protein and ash per g wet wt. over varying eel body masses. Kcal per g wet wt. from bomb calorimetry (represent energy in protein + lipid + carbohydrate) was inverse correlated with percentage water. Calculated Kcal (calculated from contents of protein + lipid) was highly correlated, but 5% higher, compared with observed Kcal from bomb calorimetry and suggest, that the factor for converting lipid to Kcal is too high and that the content of carbohydrate was not included. The results were compared with proximate analysis of domesticated rainbow trout maximal fed with high energy food. In general, rainbow trout had a lower energy content and content of lipid per g body wet wt. compared to yellow eel. In contrast to yellow eel, the content of protein per g wet wt. in rainbow trout was highly increasing with decreasing water content. We suggest that this is because the high-energy content in food and very high growth rate of domesticated rainbow trout. The constants for calculating energy from protein and lipid was discussed.