Unsustainable fishing practices have placed a heavy emphasis on aquaculture to meet the global shortfalls in the supply of fish and seafood, which are commonly accepted as the primary source of health-promoting essential omega-3 (n-3 highly unsaturated fatty acids). However, dietary fish oil is required for the production of omega-3-rich farmed fish and this commodity, in a vicious circle, is at present derived solely from wild fisheries. Decreasing global availability coupled with the highly variable price of this resource has forced the aquaculture industry to investigate the possibilities of alternative dietary lipid sources. This review attempts to compile all principal information available regarding the effects of fish oil replacement for the diets of farmed finfish, analysing the findings using a comparative approach among different cultured fish species. The review initially focuses on the present situation with regard to the production, availability and main nutritional characteristics of fish oil and the principal alternative lipid sources (such as vegetable oils and animal fats). Following this, the effects of fish oil replacement in finfish nutrition on feed quality, fish performance, feed efficiency, fish lipid metabolism, final eating quality and related economic aspects are presented and discussed.

Fish oil replacement in finfish nutrition

Chitin is the (1→4)-β-linked homopolymer of N-acetyl-D-glucosamine (Fig. 1). The individual polymer chains can be thought of as helices, as each sugar unit is inverted with respect to its neighbors. This leads to their stabilization as rigid ribbons by 03—H … 05 and 06—H … 07 hydrogen bonds. The commonest form of chitin is α-chitin. Its unit cell is of two N,N‵-diacetylchitobiose units of two chains in an antiparallel arrangement. Thus, adjacent polymer chains run in opposite directions, held together by 06—H … 06 hydrogen bonds, and the chains are held in sheets by 07 … H—N hydrogen bonds (Minke and Blackwell, 1978). This gives a statistical mixture of CH2OH orientations, equivalent to half oxygens on each residue, each forming inter- and intramolecular hydrogen bonds. This results in two types of amide groups; all are involved in the interchain C=O … H—N bonds, while half of the groups also serve as acceptors for 06—H … O=C intramolecular bonds. This extensive intermolecular hydrogen bonding leads to a very stable structure, the individual polymer chains eventually giving rise to microfibrils if allowed to crystallize (Gooday, 1983). A less common form of chitin is β-chitin, in which the unit cell is of one N,N‵diacetylchitobiose unit, giving a polymer stabilized as a rigid ribbon, as for α-chitin, by 03—H … 05 intramolecular bonds (Gardner and Blackwell, 1975). Chains are then held together in sheets by C=O … H—N hydrogen bonding of the amide groups and by the CH2OH side chains, forming intersheet hydrogen bonds to the carbonyl oxygens on the next chains (06—H … 07). This gives a structure of parallel poly-N-acetylglucosamine chains with no intersheet hydrogen bonds.

The Ecology of Chitin Degradation

This review summarizes information regarding digestion and absorption of carbohydrates in cultivated fish Relevant results of studies of digestive enzymes, eg amylase, chitinase, cellulase and brush border disaccharidases are presented Fish amylases appear to be molecularly closely related and to have characteristics comparable to mammalian amylases Whether chitinases and cellulases are endogenous enzymes of some fish species is still a matter of speculation, although recent molecular evidence, at least for chitinase seems to settle the issue in favour of endogenous sources Feed and intestinal microbes may be the source of polysaccharidases in fish feeding on nutrients-containing non-starch polysaccharides Knowledge regarding monosaccharide transport in fish intestine as interpreted from studies of brush border membrane vesicles, everted sleeves of fish intestinal sections and molecular biology is discussed Glucose transporters of the intestinal brush border show characteristics similar to those found in mammals A tabulatory presentation of experimental details and results reported in the literature regarding starch digestibility is included as a basis for discussion Although numerous investigations on digestion of starch and other carbohydrates in fish have been published, the existing information is highly fragmentary As yet, it is impossible to derive a cohesive picture on the integrated process of carbohydrate hydrolysis and absorption and interaction with diet composition for any of the fish species under cultivation The physiological mechanisms behind the species differences are not known

Carbohydrates in fish nutrition: digestion and absorption in postlarval stages

Publisher Summary
This chapter discusses the amino acids and protein requirements of fish. Proteins are the major organic material in fish tissue, making up about 65–75% of the total on a dry-weight basis. Fish consume protein to obtain amino acids. A regular intake of protein or amino acids is required because amino acids are used continually by the fish, either to build new proteins (as during growth and reproduction) or to replace existing proteins (maintenance). Inadequate protein in the diet results in a reduction or cessation of growth and a loss of weight due to withdrawal of protein from less vital tissues to maintain the functions of more vital tissues. Fish, like other animals, do not have a true protein requirement but have a requirement for a well-balanced mixture of essential or indispensable and nonessential or dispensable amino acids. Size and age, water temperature, and maintenance requirements affect the protein requirements of fishes. Various investigators have observed improved growth and feed efficiency when experimental diets for salmonids were supplemented with indispensable amino acids to simulate levels found in isolated fish protein or the respective eggs and whole-body tissue of the species being studied. The indispensable amino acid requirements of certain fish have also been shown to correlate well with the indispensable amino acid pattern of the whole-body tissue of that fish.

Amino acids and proteins

Review on the use of insects in the diet of farmed fish: Past and future

The metabolism of methionine was studied in rainbow trout fed diets containing different levels of methionine and cystine. Growth data indicated that methionine requirement was between 0.5 and 1% dry diet in the absence of dietary cystine, but 0.5% was adequate when dietary cystine was 2%. In fish fed diets deficient in sulfur-containing amino acids, elevated hepatic activities of glutathione reductase were found, whereas glutathione peroxidase and glutathione levels were unaffected. Plasma and liver concentrations (18 hours after feeding) of methionine and cystine were affected by dietary methionine, but dietary cystine had little effect. Cystine appeared to be converted to taurine in the liver. In fish injected intraperitoneally with [14COOH]- and [14CH3]methionine, over 24 hours the carboxyl group was oxidized more than the methyl group and more was incorporated into protein. However, much more of the methyl group was incorporated into the lipid fraction. The results suggest the operation of the transsulfuration pathway of methionine catabolism. Oxidation of methionine was related to its concentration in the tissues, and little affected by dietary cystine. A 28-day experiment on the metabolism of injected [14COOH]methionine showed that its turnover was slow, and much of the radioactivity was associated with protein.

Methionine Metabolism in Rainbow Trout Fed Diets of Differing Methionine and Cystine Content

The vitamin E requirement of rainbow trout (Salmo gairdneri) given diets containing polyunsaturated fatty acids derived from fish oil

The growth of rainbow trout (Salmo gairdneri) given diets containing chitin and its relationship to chitinolytic enzymes and chitin digestibility

Pyridoxine requirements of the gilthead bream, Sparus aurata☆

The ability of eels (Anguilla anguilla) to further desaturate and chain elongate linoleic acid, 18:2n-6, was studied by feeding diets containing either corn oil or a fish oil to groups of elvers for 12 wk and analyzing proportions of fatty acids in tissue lipids. Over the 12-wk period elvers given both dietary treatments increased in weight by fourfold. The proportion of arachidonic acid, 20:4n-6, present in polar lipids of elvers fed the diet containing corn oil increased from an initial value of 5% by weight to 12% by weight; the corresponding value for elvers given a diet containing fish oil, with little linoleic acid in it, was less than 4% by weight. The capacity of this fish to modify dietary linoleic acid metabolically was confirmed by examining the metabolic fate of radioactive carbon after giving [1-14C]linoleic acid orally to eels. Seven days after administration of the linoleic acid approximately 10% of the radioactivity recovered in liver fatty acids was present in trienes and tetraenes with about 4% occurring in arachidonic acid. Compositional analyses from the feeding experiment also indicated that dietary docosaenoic acid, 22:1n-11, is preferentially oxidized by eels.

C.B. Cowey

Papers

Methionine Metabolism in Rainbow Trout Fed Diets of Differing Methionine and Cystine Content

The vitamin E requirement of rainbow trout (Salmo gairdneri) given diets containing polyunsaturated fatty acids derived from fish oil

The growth of rainbow trout (Salmo gairdneri) given diets containing chitin and its relationship to chitinolytic enzymes and chitin digestibility

Pyridoxine requirements of the gilthead bream, Sparus aurata☆

Capacity of the European Eel (Anguilla anguilla) to Elongate and Desaturate Dietary Linoleic Acid