A major selling point for fish is the health value of the highly unsaturated ω-3 fatty acids in the fish oil. These oil fractions are unique in that primar- ily plankton and plants grown in the aqueous environment produce these highly desirable fatty acids. Fish consume the plankton or other smaller fish or shellfish that feed on plankton and, therefore, have a highω-3 (or n-3) content in their oil. At the present time, many aquaculture fish have low values ofn-3 fatty acids since their source of feed is a prepared diet in which the oil is of plant origin. However, including fish oil in the diets can substantially increase preferred types of fatty acids.
Although only plants and algae maken-3 fatty acids, elongation and de- saturation of HUFA occur along the food chain as fish metabolize and store these compounds. Furthermore, terrestrial plants, such as soybeans and rapeseed, containing small amounts ofn-3 fatty acids, provide only linolenic acid, C18:3,n-3, not HUFA. This fatty acid is not elongated and desaturated efficiently by mammalians (Tucker 1995). Likewise, crustaceans appear to have a limited capacity for bioconversion ofn-3 PUFA ton-3 HUFA ( Jones et al.1997; Boonyaratpalin 1998).
ω-3 fatty acids (n-3), particularly those found in high-quality fish oil, EPA (C20:5n-3) and DHA (C22:6n-3), are essential for the health, growth, and survival of larval as well as older fish and shellfish. DHA is essential for growth and optimal development of the brain and nervous system in humans as well as other animal species. Meyers (1979) speculated long ago that these long-chain HUFA might provide the intrinsic value ofArtemiafor larval diets. Certainly, a purely artificial diet for larvae must includen-3 fatty acids, preferably HUFA. Immune system functions are also enhanced byn-3 fatty acids so these lipids should improve survival as well. HUFA also seem to be required for ovarian maturation and spawning of crustacea (D’Abramo 1998). Wild larvae and broodstock ofPenaeus vannemei had lipid profiles showing 27% of the total lipids as HUFA (Pedrozzoliet al.1998), and there- fore, artificial diets, at least for marine shrimp species, should attempt to
maintain these levels. To ensure that aquaculture products are nutritionally equivalent to, or better than, wild-caught, supplemental marine oils are es- sential in feed—especially when formulations contain a significant percent- age of agricultural products (Pigottet al.1987; Pigott 1989; Tucker 1999).
Freshwater species typically contain a highern-6:n-3 ratio of fatty acids. Addi- tionally, the environmental temperature influences the levels of various es- sential fatty acids, with colder-climate fish and shellfish incorporating higher levels ofn-3 HUFA than warmwater species. Accordingly, feed should be ad- justed. The HUFA requirement for freshwater crustaceans appears to be about one-tenth that for marine species, although water temperature also plays a role (D’Abramo 1997). HUFA incorporation into tissues of all ani- mals reflects the diet.
One good source of essential fatty acids is fish meal made from fish; how- ever, the essential fatty acid content alone can be misleading since the meal from a low-fat fish (e.g., white fish) is actually much lower in total fat than that from a high-fat fish, such as herring, anchovy, and menhaden. Table 11.1 shows a comparison of HUFA between wild fish and farmed fish that have no significant HUFA in their diets. Although the wild fish have a much higher HUFA content than the farmed fish, note that farmed crayfish have about the same HUFA content as wild crayfish. This is due to the fact that cray- fish are in contact with the bottom of the ponds and tend to eat algae and other naturally growing vegetation. Wet diets, containing a high percent- age of fresh fish portions, having traditionally been fed to hatchery-raised Pacific salmon fry and fingerlings. The flesh of these fish contains a much higher HUFA content than does that of commercially raised trout fed a dry diet.
Table 11.1
Relationship betweenn-3 andn-6 Contents of Oil from Edible Portions of Wild versus Pond-Reared Shrimp, Crayfish, and Catfisha
Total
Fatty acids (%) Ratio
Source PUFA (%) n-6 n-3 n-3/n-6 HUFA/n-3
Marine shrimp 45.15 16.88 28.28 1.67 1.33
Pond-reared prawns 41.64 23.04 18.60 0.81 0.66
Wild crayfish 50.12 16.38 33.74 2.06 1.55
Pond-reared Crayfish 47.50 16.64 30.84 1.86 1.49
Wild catfish 39.77 12.13 27.64 2.54 2.00
Pond-reared catfish 26.07 15.85 10.22 0.62 0.48
aAdapted from Channugamet al.(1986).
There is also a large variation between the n-3 and the n-6 fatty acid content in wild fish. In addition to the variations between species, there are major differences within a given species or group at different periods in growth and development, especially during the spawning cycle. This most likely accounts for many of the differences in results of research involving the effects of HUFA in human and animal nutrition, especially during the early work when the product was simply cited as “fish oil.” It is extremely important that a complete analysis of any given fish oil be made prior to feeding in nutritional or clinical tests. Furthermore, over long, extended tests, the oil should be periodically analyzed to check on chemical and oxidative changes taking place over time.
The interest that has developed over the past few decades in then-3 HUFA contained in fish oil has stimulated worldwide research on methods of re- fining oil for human consumption. This has resulted in numerous projects, ranging from the recovery and stabilization of high-quality oils to clinical investigations studying the effects ofn-3 fatty acids on reducing heart and other diseases. It is interesting to note that the major efforts in producing high-quality “heart-healthy” products from fish oils are paralleled by an in- crease in the use of fish oil for fuel in the high-sea fleets processing fish.
This is especially true for the surimi operations, where the oil from fish is removed during washing of the flesh. Although the fish, such as pollock, used for surimi are low in oil content, the large volume of fish processed results in considerable oil recovery from the wash water. It has proven more economical at the present time to burn the oil with the ship fuel than to transport it to the distant, currently low-priced, markets where it is used for industrial purposes or for production of margarine.
Although there is a considerable amount of fish oil in the fish meals fed to animals, the specific feeding of fish oil has not been practiced to a large extent. Fish meal containing fish oil is fed in the diets of poultry, pigs, fish, crustaceans, ruminants, fur-bearing animals, and pets. Special efforts are being made to improve the quality of the protein and oil in fish meal so that many commercial animals, currently not fed significant amounts in their diet or needing better-quality meals, can utilize this high-protein supplement (Pigott 1997; Bimbo and Crowther 1992). These products, produced from menhaden, have proven to be beneficial for early-weaned pigs, high-yielding dairy cows, and aquaculture-raised fish.
Poultry could be a major source ofn-3 fatty acids for humans if poultry feeds include high-quality fish oil high in HUFA content. Recent studies have indicated that chickens can be a source ofn-3 fatty acids that is equal to, albeit different from, that of cod fish (Opstvedt 1985; Hulanet al.1988).
These studies have shown that significant amounts of n-3 HUFA can be incorporated in poultry diets without affecting the meat flavor. However, it
should be emphasized that only fish meal specially processed to minimize heat degradation and oil oxidation is utilizable. Many conventional meals are limited as to the amount that can be fed in poultry diets without the fish flavor, caused primarily by rancid oil and other degradation products, being transferred to the meat.
Since a large part of the world’s population consumes poultry, especially chickens, as a significant portion of the meat protein in their diets, consid- erable effort is being expended to produce low-cholesterol, high-n-3 fatty acid eggs from layer hens (Hulanet al.1989; Stadelman 1989). Up to 6%
refined menhaden oil can be added to the total diet without affecting the flavor (Yu and Sim 1987). A layer hen diet containing 3% menhaden oil was shown to increase the EPA and decrease the ratio ofn-6 ton-3 from 18 to 3 (Oh et al.,1988). Other experiments have shown that regular men- haden oil, stabilized with antioxidants, could be fed to layer hens at a level of 3% of the diet without causing a “fishy” flavor (Hargiset al.1991).
11.2.2.2. n-6/n-3 Ratios
The importance of the ratio ofn-6- to -n-3 fatty acids has only recently been seriously addressed in human nutrition. The dramatic increase in dietary vegetable oils (n-6) during the past half-century has resulted in increased immunosuppression and inflammation. Human diets are now 20–40:1n-6:n-3, whereas for hundreds of thousands of years the ratios had been 1–4:1. Lim et al.(1997) showed that n-6 and n-3 fatty acids are es- sential for juvenileP. vannemei,butn-3 fatty acids promoted faster growth than n-6 fatty acids. The n-3 HUFA had better growth-promoting effects than linolenic acid. Preferential incorporation and conservation of HUFA in polar lipids of crustacean tissue have been demonstrated (D’Abramo 1998). Perhaps one reason that alternate sources of protein from plants, with their differingn-6:n-3 content, have limited usefulness, especially for marine species of crustacea, is due to an alteration of then-6:n-3 ratio.
11.3