220 FISH HAICIIKRV MANAGEMENT 



No carbohydrate requirements have been estabHshed for catfish; how- 

 ever, carbohydrates can spare protein in catfish feeds. In the absence of 

 adequate dietary carbohydrates or fats, catfish make inefficient use of 

 dietary protein to meet their energy and other metabolic needs. In channel 

 catfish, lipids and carbohydrates appear to spare protein in lower-energy 

 feeds but not in higher-energy feeds. 



Fiber is an indigestible dietary material derived from plant cell walls. 

 Fiber is not a necessary component for optimum rate of growth or nutrient 

 digestibility in channel catfish production rations. Fiber levels as high as 

 21% reduce nutrient intake and impair digestibility in feeds for channel 

 catfish. Fiber in concentrations of less than 8% may add structural integrity 

 to pelleted feeds, but larger amounts often impair pellet quality. Most of 

 the fiber in the feed ultimately becomes a pollutant in the water. 



Lipid Requirements 



Lipids comprise a group of organic substances of a fatty nature that in- 

 cludes fats, oils, waxes, and related compounds. Lipids are the most con- 

 centrated energy source of the food groups, having at least 2.25 times more 

 energy per unit weight than either protein or carbohydrates. In addition to 

 supplying energy, lipids serve several other functions such as reserve ener- 

 gy storage, insulation for the body, cushion for vital organs, lubrication, 

 transport of fat-soluble vitamins, and maintenance of neutral bouyancy. 

 They provide essential lipids and hormones for certain body processes and 

 metabolism, and are a major part of reproductive products. 



Although each fish tends to deposit a fat peculiar to its species, the diet 

 of the fish will alter its type. The fat deposited tends to be similar to the 

 fat ingested. The body fat of fish consuming natural foods contains a high 

 degree of polyunsaturated (soft) fats similar to those in the food. Because 

 natural fats are soft fats that are mobilized and utilized by the fish more ef- 

 ficiently than hard (saturated) fats, soft fats are beneficial for efficient pro- 

 duction and fish health. Preliminary studies have indicated that some hard 

 fats can be used by warmwater fish. 



The effect of water temperature on the composition of the body fat of 

 fish is difficult to define clearly due to its influence on the digestibility of 

 hard and soft fats. Soft fats are digested easily in both warm and cold wa- 

 ter, but hard fats are digested efficiently only in warm water. Fish living in 

 cold water have body fats that are highly unsaturated with a low melting 

 point. These fish are able to more readily adapt to a low environmental 

 temperature. 



Factors to be considered in evaluating dietary lipids for fish feeds in- 

 clude digestibility, optimal level in the feed, content of fatty acids essential 



