Basal feeds, or energy feeds, are low-protein, high-energy feed ingredi- ents. The upper limit for protein content of basal feeds is 20%, although most are in the 10–17% range. Grains are generally 68–72% starch, with the exception of oats, and about 10–12% protein. Domestic animals can digest about 95% of the starch in grains, hence their classification as energy feeds.
The digestibility of the carbohydrates in grains is highly variable among fish. Carnivorous species, such as salmonids, derive very little energy from unprocessed plant starch. Omnivorous species, such as catfish, and herbivo- rous species, such as some carp, derive a high amount of energy from grain starch, providing that it is cooked.
By convention in animal nutrition, energy feeds cannot contain over 18% fiber (Crampton and Harris 1969). Otherwise, they are classified as roughages. Fiber is indigestible by carnivorous fish, while omnivorous and herbivorous fish are able to digest fiber to varying, limited degrees. The fiber content of grains is about 6%, although oats and barley contain higher levels, at least hulled varieties.
Numerous by-products of grain processing are used in animal feeds and are also classified as basal feeds. Wheat milling, which produces flour as its main product, also produces wheat shorts, wheat bran, wheat mill-run, feed flour (second-class flour), and wheat middlings, all of which can be used in fish feeds. By-products of oats, corn, and other grains are also potential fish feed ingredients. Due to the removal of starch (flour), these products contain higher protein, fiber, and fat levels than the grain from which they were derived.
Factors other than nutrient content influence the use of basal feeds in animal diets. The most important factor is cost, which is usually relatively low. Thus, after the nutrient requirements have been met, basal feeds can be used to fill out the energy needs in animal feed formulation. Basal feeds also have excellent binding properties and can help to hold feed pellets together in both dry and semimoist diets. Finally, they are relatively indigestible, and this quality is useful in animal production when a reduced rate of growth at a normal feeding level is desired. Rarely does a similar situation arise in fish culture.
A large number of feed ingredients other than those listed here may be suitable for use in fish feed formulation. As mentioned earlier, over 18,000 individual feed ingredients have been identified and classified throughout the world. Although most of these ingredients will never be used in fish feeds, some are important feed constituents in developing countries. Fish nutritionists continue to identify potential fish feed ingredients and charac- terize their chemical composition and nutritional value to fish.
9.3.3.2. Fats and Oils
Fats and oils are lipid sources, fats being the term for lipids that are solid at room temperature and oils being the term for lipids that are liquid at room temperature. The melting point of a lipid depends on its fatty acid composition. Lipids containing a high proportion of saturated fatty acids have a lower melting point than those containing a high proportion of unsaturated fatty acids. Thus, tallow is termed a fat, while plant or fish lipids are called oils. In either case, they are concentrated energy sources for fish and animals, having 2.25 times as many calories per gram as carbohydrates, a result of their structure.
It used to be thought that fish could not digest fats, because they were solid at the water temperature at which most fish are cultured. However, research has shown that fats are digestible to fish, although slightly less so than oils (Austreng and Refstie 1979; Ellis and Smith 1984). The main factors determining which lipid source to use in fish feed formulations are the fatty acid composition of the lipid source and its physical characteristics at ambient temperatures, which dictate how it must be stored and handled at the feed mill.
A variety of commercially available fats and oils is suitable as ingredients for fish diets. Salmonid diets normally include fish oils, such as herring, pollock, menhaden, anchovy, and capelin. Plant oils, such as soybean, corn, and cottonseed, and animal fats, such as tallow, lard, and poultry fat, may also be used as long as the diet contains sufficient levels of essential fatty acids (Takeuchi and Watanabe 1982). Fats and oils are chosen using the same criteria as for other ingredients: price, availability, nutritional value,
Table 9.7
Quality Standards for Fish Oil Required for Salmonid Dietsa
Component Level
Iodine value Report value
Peroxide value <5 mEq/kg Anisidine value <10 Pesticides (total) <0.4μg/g
PCBs <0.6μg/g
Nitrogen <1.0%
Moisture <1.0%
Antioxidant <500μg/g
aOntario Ministry of Natural Resources (1998).
and quality, as defined by chemical tests. Since fats and oils are susceptible to oxidative and hydrolytic degradation, specifications on the percentage of free fatty acids and products of oxidation, such as malonaldehyde and per- oxides, are generally made. Limits on water content, unsaponifiable matter content, and insoluble matter are also often specified. Occasionally, refined fish oils are available on the market; oils are refined to reduce free fatty acid values and nonfat material. Even though these oils are within specifications for fish feed use, they may be low in naturally present antioxidants, which can result in a shortened induction period prior to the onset of oxidation. This can be prevented by adding antioxidants. Antioxidants are usually added to fish oils during production to prevent oxidation, but buyers of oil for fish feed use should make certain this had been done. Ethoxyquin is the most common antioxidant used in fish oils. In the United States feeds cannot contain over 150 ppm ethoxyquin, from all sources. Typical specifications for fish oil for fish feed use are listed in Table 9.7.
Another concern with fats and oils is the possible presence of organic con- taminants, many of which are lipophillic. Several published values for poly- chlorinated biphenyls (PCBs) and 1,1-dichloro-2, 2-bis(p-chlorophenyl) ethylene (DDE) in fish feeds are available (Crockett et al. 1975; Gruger et al. 1976). While none of the published values approach toxic levels, the levels may be of practical significance to scientists conducting contaminant- exposure studies (Mac et al. 1979). More recently, concerns about dioxin contamination of feed ingredients have been raised. Specifically, certain clays used in animal feeds were found to contain dioxin. Rising concerns about the purity and safety of food coupled with improvements in analyti- cal testing for organic contaminants of feeds and foods will likely result in
more rigorous testing in the future. Since feed ingredients are the principal source of potential contamination of feeds and foods, we can expect increas- ing surveillance of feed ingredients and changes in ingredient processing and use patterns to result.