excretion through the gills (ZE) and urine (UE). The difference between ME and energy recovered as growth and/or reproductive products (RE) is energy lost as heat (HE). Heat loss occurs primarily by two processes: the heat increment of feeding (HiE) and maintenance heat loss (HEm).

The HiE is the increase in heat production subsequent to ingestion of feed. The factors contributing to HiE are the digestion and absorption processes (HdE), the transformation and interconversion of the substrates and their retention in tissues (HrE), and the formation and excretion of metabolic wastes (HwE). The main biochemical basis for HiE in mammals and birds is the energy required for the ingested amino nitrogen (N) to be deaminated and excreted (Kleiber, 1975); however, this represents less of an energy loss in fish because they can eliminate and products of protein metabolism (ammonia, bicarbonate, and carbon dioxide) without the need to synthesize urea, uric acid, or other similar compounds. Energy expenditures associated with diet ingestion and digestion are small compared with that associated with metabolic work (Brody, 1945). This conclusion has been reinforced by the observation that intravenous infusion of amino acids increases heat production to the same extent as does the oral administration of the amino acids (Benedict and Emmes, 1912; Borsook, 1936). HiE depends to a large extent on the balance of dietary nutrients and the plane of nutrition (Brody, 1945) and, in fish, the water temperature (Cho and Slinger, 1979). Thus, measurement of HiE for balanced feeds is more meaningful than measurement of the HiE of individual feed ingredients, because the metabolic fate of absorbed nutrients depends on the mixture absorbed and, hence, the variety of metabolic processes that are possible.

The HiE in fish is greater for diets with a high protein content than for diets with a low protein content (Cho, 1982). In mammals and birds, however, the effect of high dietary protein on heat increment is even more marked, partly because of the energy expenditure during synthesis of urea or uric acid from the deaminated nitrogen. The energy cost of synthesis for urea and uric acid is 3.1 and 2.4 kcal/g N, respectively (Martin and Blaxter, 1965). In contrast, ammonia is the primary nitrogenous waste product of protein catabolism in fish (Goldstein and Forster, 1970). Because this form of nitrogen can be readily released into the water, energy expenditure on urea or uric acid synthesis is not needed (Cowey, 1975). Cho et al. (1982) found that HiE for rainbow trout at 15°C was 5 to 15 percent of the gross energy consumed (IE) and fell as the ratio of protein to energy decreased. The HiE for livestock can be as much as 20 to 30 percent of the IE (Farrell, 1974; National Research Council, 1984). Thus, because of the lower heat increment of fish, the net energy (NE), which is the energy that is useful to the animal for maintenance and growth, in production diets is higher for fish than for warm-blooded animals.

Maintenance energy (HEm) is that required to maintain those functions of the body immediately essential to life. A major portion of this maintenance energy is spent for basal metabolism (HeE), such as respiration, transport of ions and metabolites, body constituent turnover, and circulation. A smaller portion is spent for voluntary or resting activity (HjE) and, in the case of homeothermic animals, thermoregulation of body temperature. Since fish do not regulate body temperature and they expend less energy in maintaining position in the water than do terrestrial animals in maintaining their posture, the HEm requirement of fish is lower than for homeotherms. The fasting heat production (HEf) is an approximation of the HEm. Cho and Kaushik (1990) measured oxygen consumption of fasting rainbow trout weighing 96 to 145 g at 15°C and calculated their HEf, in kcal/fish/day to be 8.85 W^0.82 where W is body weight in kilograms. Smith (1989) reported an HEf value of 4.41 W^0.63 for rainbow trout weighing 4 to 50 g at 15°C where fasting heat production was measured directly by placing the fish in a calorimeter. Brett and Groves (1979) recommended the exponent 0.8 for metabolic body size for fish. When these HEf values for fish are compared with 70 W^0.75 for mammals and 83 W^0.75 for birds (Brody, 1945), it is apparent that the fasting heat production of fish is much lower. The maintenance energy requirements of fish are one-tenth to one-twentieth of those of homeothermic animals of similar size in a thermoneutral environment (Brett, 1973). The lower maintenance requirement for fish means that the percentage of net energy that is not dissipated as heat but retained within the body as new tissue or recovered energy is greater.

The energy content of a diet depends on its chemical composition, with the mean values of heat of combustion of protein, lipid, and carbohydrate being 5.64, 9.44, and 4.11 kcal/g, respectively. However, the chemical makeup of the diet influences only its heat of combustion, or gross energy, and yields no information on whether the energy and nutrients are available to fish through the digestive process. Prior to formulating diets, therefore, it is necessary to know the bioavailability of the energy in the feedstuffs for the animal being fed.

Available energy values for feedstuffs for fish have been determined on a DE and ME basis. ME, where applicable, is a more exact measure of the energy value for a complete diet that becomes available for metabolism by the animal. Practically, ME offers little advantage over DE in evaluating useful energy in feedstuffs for fish because FE accounts for most of the excretory losses. Energy losses through ZE and UE by fish are smaller than nonfecal energy losses by mammals and birds, and they do not vary among feedstuffs as much as do FE losses. Furthermore, determining ME values with fish is difficult because of the need to force feed

Front Matter (R1-R12)

Overview (1-2)

1. Dietary Requirements (3-32)

2. Other Dietary Components (33-37)

3. Antinutrients and Adventitious Toxins (38-42)

4. Digestibility and Absorption (43-48)

5. Diet Formulation and Processing (49-54)

6. Feeding Practices (55-61)

7. Nutrient Requirements Table (62-63)

8. Composition of Feed Ingredients (64-71)

References (72-94)

Appendix (95-102)

Authors (103-104)

Index (105-116)