The bioenergetics of grass carp, Ctenopharyngodon idella (Val.): energy allocation at different planes of nutrition

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Abstract
Complete energy budgets were constructed for 19 grass carp, Ctenupharyngudon idella (Val.), held individually in a respirometer for a month. The fish were fed one of four diets or starved. Diets varying in the proportions of protein, lipid and carbohydrate were described as high protein (HP), high carbohydrate (HC) or high lipid (HL). A fourth diet (LM) was made from dried duckweed, Lemna spp., to provide a more natural diet. Fish were fed and faeces collected daily and oxygen consumption was measured continuously over the month that each experiment lasted. Excretion of ammonia and urea was measured on several days. The total energy lost via nitrogenous waste was calculated using an average daily ammonia quotient (AQ).For growing fish between 50 and 61% of consumed energy was lost via respiration. Energetic losses via nitrogenous wastes were highest on the HP diet (4.7%) and lowest on the HC diet (3.1%). Faecal loss washigheston the HL diet (19.4%)and lowest on the HP diet (10.2%). Over a month of starvation, 32.5% of energy requirement was met by the respiration of protein and 3.2% of the total energy lost was via nitrogenous waste. Fish fed zero or sub‐maintenance rations tended to respire lipid in preference to protein whereas fish fed super‐maintenance rations accumulated lipid. Protein retention was proportionally highest on HP (48% of total energy retained as growth) and lowest on HC (32%) and HL (30%). This reflected the accumulation of lipid on both the high carbohydrate and high lipid diets. The partitioning of metabolizable energy (ME) was investigated and 0.45 (HL), 0.59 (HP) and 0‐67 (HC) kJ ME.kJ‐1 retained were lost via respiration.