Fish Nutrition
Most of our knowledge concerning fish dietary requirements come from experimental nutrition studies 
conducted on cultured species, primarily salmonids. These studies have demonstrated the relative importance of diet-aryl proteins, lipids, and carbohydrates for growth (anabolism) and for energy to run the bodily machinery (catabolism). Proteins, which con-sits of chains of amino acids, seem to be essential mainly for growth, al-though they may also be used for catabolic functions. The importance of proteins for growth has been shown in numerous nutritional studies that omit proteins containing amino acids the fish are not capable of synthesizing themselves. For example, Halve (1957) fed experimental groups of Chinook salmon (Oncorhynchus tshawytscha) diets devoid ofsingle amino acids and compared their growth rates with control animals fed diets containing all the amino acids. He found that their growth rates were greatly reduced because new structural proteins (for muscle, bone, etc.) could not be synthesized when one or more amino acids composing the specific protein chain were missing. Missing amino acids can also provoke developmental vertebral abnormalities such as scoliosis and lord sis (Halve and Shanks 1960). These no synthesizable amino acids therefore become “essential” in the diet of the fish. Ten amino acids which have been shown to be essential for fish are argentine, histidine, isoleucine, leonine, lysine, methionine, phenylalanine, heroine, tryptophan, and valise. The quantities of the various required amino acids needed, however, vary among species, and excessive amounts of anyone acid may also be detrimental to growth and survival.
In wild fish, proteins are often an important source of energy for meeting metabolic demands. For example, rainbow trout (Salmon gaird-neri) in the wild feed largely on aquatic and terrestrial invertebrates, making protein a high percentage of their natural diet, far beyond what is needed for growth. In cultured fishes, however, the protein fraction of the diet usually comes from fish meal and is a comparatively expensive part of the feed. To minimize their monetary costs of operation, fish culturists include protein in quantities sufficient only for anabolic processes and substitute lipids or, especially, relatively inexpensive car-bohydrates for a source of energy. A significant energy cost is incurred in breaking down (hydrolyzing) the large, complex protein molecules. This cost is termed the specific dynamic effect (SDE) or specific dynamic action (SDA) and increases with the amount of protein in the diet.
Thus, Schaller and Wising (1976) calculated that 12.6% to 16.1% ofthe ingested energy was used by bluegills to digest and assimilate diets containing 23.9% to 45.3% protein, respectively.
Carbohydrates and lipids constitute the other available energy sources in foods. In natural aquatic environments, lipids are found both in animal and plant sources, while carbohydrates are found almost exclusively in plants. The low digestibility of carbohydrates by carnivores such as trout and salmon contributes to the low energy value gained from their ingestion. A salmon extracts only 1.6 kcal of energy from1 g of carbohydrate fed, while gaining 3.9 kcal/g for ingested protein and 8.0 kcal/g for lipids. The salmon culturist must thus balance the low feed costs of carbohydrate sources (e.g., grains and cereals) with their low nutritional value. Of the carbohydrates, monosaccharide’s are the most digestible, followed (in order) by disaccharides, simple poly-saccharine, dextrin’s, cooked starches, and raw starches (Halve 1976).Some herbivorous and omnivorous fishes, such as anchovies (Anchor),sea catfish (Arius fells), and channel catfish (Ictalurus punctatus) may utilize gut microbes to break down the plant structural carbohydrate, cellulose (Stickney and Shum way 1974). The bacteria having this cellulose activity are either maintained in the gut or regularly brought in with ingested detritus (Preys and Blaszczyk 1977).
Lipids represent a rich source of energy for fishes in general. Be-sides their high specific energy value (8.0 kcal/g), they are also almostcompletely digestible (Halve 1976). The high lipid content of a diet consisting of small fish maximizes growth by sparing the ingested pro-teen for tissue synthesis. For example, rapid growth rates are typically achieved by predaceous fishes such as the mackerels, billfishes, salmon, pikes, and sharks. Besides an energy source, lipids provide essential fat-tee acids. Fatty acids are used in the construction of fats or oils (tri-glycosides) to be stored by a fish for use as an energy source at a later time. A classic example is the Pacific salmon (Oncorhynchus), which accumulates lipids at sea and expends them while fasting during migrations upstream to spawn. Experiments with catfishes (Ictalurus) have shown that body lipids synthesized by fish for energy storage parallel those ingested in terms of saturation (completeness of hydrogen bond-in of constituent fatty acid carbon chains) (Andrews and Stickney1972).
The relative importance of lipids and proteins as energy sources is also shown by their mobilization by fishes during periods of starvation, which can be a regular occurrence in the life cycle of many fishes. For example, winter flounder (Pseudopleuronectes Americans) inhabiting coastal Maine waters fast while in deeper water during the period January to May (Bridges et al. 1976). Pacific salmon as well as Atlantic salmon and steelhead trout fast during their spawning migration. As no carbohydrates, protein, or fat are taken into the body during a fast, the fish must use compounds stored in body tissues. Seitz (1971) showed that bluegill utilize body protein and, especially, fat to meet body energy demands when fasting. The quantities of body fat and protein are sig-nificantly reduced in the starved fish, while inorganic content (ash) re-mains about constant. Protein depletion is presumably accomplished by the high concentration of proteolysis enzymes found in fish muscle (Siebert et al. 1964). Losses in body fat or protein as seen in bluegill are rarely reflected in significant body weight changes in the fish. In-stead, the metabolized fat or protein is replaced by water to make up the body weight difference. For example, the whole body water con-tent of sockeye salmon increased from about 60% to 77% during the spawning migration, while the sum percentage of lipid plus water was approximately constant at 80% (Idler and Biters 1959). The familiar documentation of ascending salmon possessing an atrophied gut but still striking a fisherman’s bait represents an interesting contradiction. Even if food were swallowed, the degeneration of the digestive tract and the significant decrease in gastric enzyme secretion indicate that very little of the food could be digested.
At the other end of the feeding spectrum, it is of interest to know what happens when fish have unlimited food available to them. In their investigation of unrestricted feeding in juvenile rainbow trout, Graytonand Beamish (1977) found that the trout held at 10°C would only consume just fewer than 4% of their wet body weight per day of dry, prepared trout pellets. The trout would consume this quantity whether they were offered the pelletized food in unlimited quantities twice daily or up to six times a day. As one would expect, growth rate also did not vary with feeding frequencies from two to six feedings per day. In contrast, Balloon (1977) describes deep-bodied, obese body shapes associated with an extreme abundance of food. Deep-bodied almonds (including rainbow trout), pikes (e.g., Sox locus), carps, and others have been described in cultured or natural environments where food is very abundant. Fish tend to grow through-out their lives, but increase more in girth rather than length toward the ends of their lives.