vitamin E has little antioxidant activity in the diet. More vitamin E is required alone than when used in combination with antioxidants. Murai and Andrews (1974) showed that the commercial antioxidant ethoxyquin (1,2-dihydro-6-ethoxy-2,2,4-trimethyl-quinoline) could physiological replace vitamin E in channel catfish. Lipoid degeneration of liver and other pathologies have been associated with the feeding of rancid fat and/or the absence of vitamin E from the diet (Smith, 1979; Moccia et al., 1984; Tacon, 1985).

Synthetic antioxidants used in fish feeds are ethoxyquin, BHT (3,5-di-tert-butyl-4-hydroxytoluene), BHA (2(3)-tertbutyl-4-hydroxyanisole), and propyl gallate. Several excellent reviews of antioxidant types and mechanisms have been published (Uri, 1961; Stuckey, 1968; Porter, 1980). The maximum concentration of BHA and BHT permitted by the FDA is 0.02 percent of the fat content; for ethoxyquin, it is 150 mg/kg diet (21 C.F.R. § 573.380, 582.3169, 582.3173 [1987]).

Many plants and animals contain a variety of natural pigments that impart yellow, orange, and red colors to the flesh, skin, and eggs of fish. One of the most important groups of natural pigments in the plant and animal kingdom is the carotenoids. Fish and birds use oxygenated carotenoids (xanthophylls) for pigmentation of skin, flesh, and plumage. Fish cannot synthesize these pigments; therefore, they must be present in the diet. In salmonids, two oxycarotenoids, astaxanthin (3,3'-dihydroxy-4,4'-diketo-β-carotene) and canthaxanthin (4-4'-diketo-β-carotene) are responsible for the red to orange coloring of the flesh, skin, and fins. Astaxanthin is the main carotenoid pigment of wild salmonids, and is derived mainly from zooplankton. Feedstuffs of plant origin contain pigments that do not produce the desired color of salmon flesh. The major plant carotenoids are lutein (3,3'-dihydroxy-a-carotene) and zeaxanthin (3R,R'-β,β-carotene-3,3'-diol), as found in alfalfa, yellow corn, and algae. Lutein produces a yellow color whereas zeaxanthin imparts a yellow-orange color. Carotenoid concentration of some animal and plant sources are presented in Tables 2-1 and 2-2.

The retention of carotenoids in tissues depends on absorption, transport, metabolism, and excretion of these compounds (as reviewed by Torrissen et al., 1989). The digestibility of astaxanthin found in yeast and shrimp waste meal is low. However, ensiling of shrimp by-products improves the digestibility of astaxanthin by degrading the chitincalcium-protein-carotenoid complex in shrimp shells (Torrissen et al., 1981). Free astaxanthin is absorbed more efficiently than the astaxanthin ester (Torrissen and Braekkan, 1979; Schiedt et al., 1985). It appears that the rate of hydrolysis of the astaxanthin ester to free astaxanthin in the digestive tract of salmonids is limited. Approximately 90 percent of astaxanthin in fish flesh is located in free form, while the ester form predominates in skin. Salmonids are not able to oxygenate carotenoids, but deposit ingested oxygenated carotenoids without modification. Wide differences in the accumulation of carotenoids may be due to the differences in absorption of these compounds. Apparently, absorption is enhanced by the incorporation of hydroxyl groups into the carotene skeleton because astaxanthin is deposited at significantly higher concentrations than canthaxanthin in both Atlantic salmon and rainbow trout (Torrissen, 1986, 1989; Choubert and Storebakken, 1989). In salmonids, the absorption of astaxanthin and canthaxanthin is 10 to 20 times more efficient than lutein and zeaxanthin, while chickens absorb zeaxanthin at three times the rate of astaxanthin (Schiedt et al., 1985).

Yellow pigment deposition in the flesh of channel catfish, which is produced by zeaxanthin and lutein (Lee, 1987), is considered undesirable. Lee (1987) found that a concentration of 0.6 g carotenoid/g of flesh produced a distinguished yellow color of the fillet. A discernible concentration of carotenoid was deposited in catfish flesh from feeds containing 11 mg xanthophyll/kg.

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)