Skin pigmentation is important in cultured yellowtail and red sea bream. These fish convert dietary astaxanthin largely into tunaxanthin and deposit it in their skin. Goldfish and fancy red carp are similar to the chicken in their absorption preference: zeaxanthin-astaxanthin-lutein. Hata and Hata (1972, 1973, 1976) showed that the yellow pigment, zeaxanthin, is readily metabolized to astaxanthin in goldfish and fancy red carp, which imparts red coloration. Goldfish metabolize little β-carotene and no lutein to astaxanthin (Hata and Hata, 1972).

The function of carotenoids other than as precursors of vitamin A in fish is not well defined and mostly speculative (Tacon, 1981). Although the mobilization of carotenoids from the flesh to skin and ovaries of salmonids during maturation is well documented, their role in reproduction is not clear. Schiedt et al. (1985) reported a biological function of astaxanthin, canthaxanthin, and zeaxanthin as vitamin A (retinol and 3,4-dihydroretinol) precursors for vitamin A-depleted rainbow trout.

Binders are incorporated into fish feeds to improve stability in water, increase pellet firmness, and reduce the amount of fines produced during processing and handling. Among the most widely used binders are sodium and calcium bentonites, lignosulfonates, hemicellulose, carboxymethylcellulose, alginate, and guar gum. More recently, some inert polymeric binders have been introduced, but limited information is available on their composition or toxicity to commonly cultured fish. Cereal grains provide starch that, when gelatinized, gives a durable, water-stable pellet. Certain feed ingredients such as whey, wheat gluten, pregelatinized starches, and molasses will permit the production of good-quality pellets. Most binders are considered to be inert and have limited or no nutritional value. However, incorporation of alginate and guar gum in rainbow trout diets reduced feed intake, increased moisture content of feces, and lowered the digestibility of protein and lipids (Storebakken, 1985). Wood et al. (1954) showed that carboxymethylcellulose at 2 percent in the diet of trout caused no growth depression.

The primary modes of feed detection by fish are through olfaction or sight, but the taste of the item is the key factor in determining whether the item is swallowed or rejected (Adron and Mackie, 1978). There appears to be a well-defined and species-specific tuning of the taste receptors of fish for the particular cues present in their feed items (Goh and Tamura, 1980). Many researchers and feed manufacturers have attempted to add substances to fish feeds to enhance palatability and feed acceptance. This focus has taken particular importance in the production of larval and starter feeds, where feed acceptability is a major concern.

Carr (1982) identified four major characteristics of feeding stimulants for fish that were derived from animal tissues: (1) they have a low molecular weight (<1,000), (2) they contain nitrogen, (3) they are nonvolatile and water-soluble, and (4) they are amphoteric (have both acid and base properties simultaneously). Several substances or groups of substances for which these generalizations apply, such as amino acids, betaine, and inosine, have improved feeding behavior in carnivorous and omnivorous species (as reviewed by Atema [1980], Carr [1982], Mackie [1982], Adams and Johnsen [1986a], Rumsey [1986]). Harada (1989) has shown that some dipeptides elicit a greater feeding response than either of the constituent amino acids presented alone for abalone. Few data exist on feeding stimulants for herbivorous species, but in four studies using Zillii's tilapia (Adams and Johnsen, 1986a,b; Johnsen and Adams, 1986; Adams et al., 1988), organic acids along with certain amino acids were found to be stimulatory. Feeding was stimulated by the organic acid, dimethyl-β-propiothetin, in goldfish, common carp, and tilapia (Nakajima et al., 1989).

When data on the effectiveness of the various feeding stimulants containing amino nitrogen are considered, a pattern seems to emerge relating to the feeding behavior of the fish and the type of compounds that are stimulatory. In general, carnivores show the greatest positive response to alkaline and neutral substances, such as glycine, proline, taurine, valine, and betaine, while herbivores respond more