dietary valine was increased. No changes in plasma valine concentration occurred until valine in the diet reached the required concentration, after which the plasma valine increased about 2.5-fold. In contrast, rainbow trout showed a high tolerance for dietary leucine; no growth depression occurred with concentrations as high as 9.2 percent. Even with excessive dietary leucine concentrations (13.4 percent), which were overtly toxic, the concentrations of free valine and isoleucine in plasma, liver, and muscle were not depressed (Choo, 1990).

Another interaction characteristic of some homeotherms, and referred to as an imbalance, occurs when diets are supplemented with the second most limiting amino acid, or with all indispensable amino acids other than the first limiting amino acid. This leads to a fall in the concentration of the first limiting amino acid in the blood and eventually to reduced feed intake even though retention of the first limiting amino acid is not affected. No data are available on such interactions in fish. Nevertheless, oversupplementation with the second most limiting amino acid should be avoided as it may exacerbate a primary deficiency. There is considerable information available on amino acid interrelationships in mammals. Further information on these relationships can be found in Czarnecki et al. (1985), Baker (1987), and May et al. (1991).

Dietary lipids are important sources of energy and of essential fatty acids (EFA) that are needed for normal growth and development. They also assist in the absorption of fat-soluble vitamins. Dietary lipids, mainly in the form of triacylglycerols, are hydrolyzed by digestive enzymes to a mixture of free fatty acids and 2-monoglycerides. These compounds are then absorbed and either used for the synthesis of various cellular components or catabolized for energy.

Dietary lipids contain both saturated and unsaturated fatty acids. Fatty acids may be designated by numbering either from the methyl or carboxyl terminal. The notation from the methyl terminal is most convenient for many nutritional purposes and is used here. It involves three numbers given in sequence, the first denoting the number of carbon atoms; the second, following a colon, the number of double bonds; and the third, designated as (n-) indicates the number of carbon atoms between the methyl terminal and the first double bond. The term polyunsaturated fatty acid (PUFA) normally refers to fatty acids with 18 or more carbon atoms and two or more double bonds.

In common with other vertebrates, fish cannot synthesize either 18:2(n-6) or 18:3(n-3) de novo. Hence one or both of these fatty acids must be supplied preformed in the diet, depending on the EFA requirements. In addition, fish vary considerably in their ability to convert 18-carbon unsaturated fatty acids to longer-chain, more highly unsaturated fatty acids of the same series (Owen et al., 1975). The EFA requirement of the fish is thus related, to some extent, to their ability to modify these fatty acids metabolically.

The quantitative EFA requirements of several fish species are summarized in Table 1-14. A major difference appears to exist between freshwater and stenohaline marine fish (those unable to withstand a wide variation in water salinity). In general, freshwater fish require either dietary linoleic acid, 18:2(n-6), or linolenic acid, 18:3(n-3), or both, whereas stenohaline marine fish require dietary eicosapentaenoic acid (EPA), 20:5(n-3), and/or docosahexaenoic acid (DHA), 22:6(n-3).

Among the freshwater species, the ayu, channel catfish, coho salmon, and rainbow trout require 18:3(n-3) or EPA and/or DHA. Chum salmon, common carp, and Japanese eel require an equal mixture of 18:2(n-6) and 18:3(n-3); whereas, Nile tilapia and Zillii's tilapia require only 18:2(n-6) for maximum growth and feed efficiency. Striped bass, however, require n-3 PUFA and cannot chain elongate 18:3(n-3) (Webster, 1989; Webster and Lovell, 1990).

The principal gross signs of EFA deficiency reported for various fishes are dermal signs (fin rot), a shock syndrome, myocarditis, reduced growth rate, reduced feed efficiency, and increased mortality (Castell et al., 1972; Takeuchi and Watanabe, 1977a,b; Takeuchi et al., 1980; Satoh et al., 1989). Essential fatty acid deficiency has also been shown to reduce the reproductive performance of common carp (Shimma et al., 1977), rainbow trout (Watanabe, 1982; Watanabe et al., 1984c; Leray et al., 1985) and red sea bream (Watanabe et al., 1984a,b).

In fish species that can further desaturate and chain elongate 18:2(n-6) or 18:3(n-3), an absence of either of these fatty acids in the diet leads to the desaturation and chain elongation of oleic acid, 18:1(n-9), to 20:3(n-9), which is characteristic of an EFA deficiency in many terrestrial animals. Thus when EFAs are deficient, increased concentrations of 20:3(n-9) are incorporated into tissue polar lipids in place of 20:4(n-6), 20:5(n-3), or 22:6(n-3). Castell et al. (1972) suggested that the ratio of 20:3(n-9)/20:5(n-3) in polar lipids from the liver of rainbow trout might be a useful index of EFA status. By analogy with mammals, the diet is considered satisfactory with respect to EFA if this ratio is not greater than 0.4.

Watanabe et al. (1983) have reported that n-3 PUFAs such as EPA and DHA, are required for normal growth and development of ayu and red sea bream larvae. High mortalities and abnormalities, such as underdeveloped swim bladder and scoliosis, have been observed in red sea bream larvae reared on rotifers and Artemia spp., either devoid of n-3 PUFAs or containing only low concentrations of n-3 PUFAs

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)