Bioenergetics and nitrogen balance of the European eel (Anguilla anguilla) fed at high and low ration levels

European eel (Anguilla anguilla) were fed either high or low ration levels (1.33 and 0.48% body weight.day(-1)). Eight periods of continuous 24-h respirometry in flow-through conditions were conducted over 70 days to simultaneously examine nitrogen and energy budgets without resorting to the calculation of parameters by difference. Food and oxygen consumption; ammonia, urea, and nitrate-nitrite excretion; and growth production were measured. Nitrogen budgets successfully accounted for 89.6-101.2% of the consumed nitrogen. Energy budgets accounted for 53.4-92.3% of the consumed energy. From the ammonia quotients (AQ), 54.4-57.7% of the available protein was oxidized at high ration, and 59.7-133.5%, at low ration level. To satisfy maintenance requirements, 0.28 mg N.g(-1).day(-1) was required. At 25 degrees C eels were calculated to have a rate of standard metabolism of 83.3 J.g(-1).day(-1), which is comparable with the range published for other species at this temperature. These data suggest that eels do not have a particularly low metabolic rate, as is commonly assumed in the eel rearing industry, and that the relatively inactive lifestyle associated with the eel masks a metabolic rate in common with other, more active, fish.