218 FISH HATCHF.RY MANAGEMF.NT 



the digestibility and metabolism of carbohydrates by fish. All of the neces- 

 sary enzymes for digestion and utilization of carbohydrates have been 

 found in fish, yet the role of dietary carbohydrates and the contribution of 

 glucose to the total energy requirement of fishes remain unclear. 



There is little carbohydrate (usually less than 1.0"/ii of the wet weight) in 

 the fish body. After being absorbed, carbohydrates are either burned for 

 energy, stored temporarily as glycogen, or formed into fat. Production of 

 energy is the only use of carbohydrates in the fish system. No carbohydrate 

 requirements have been established for fish because carbohydrates do not 

 supply any essential nutrients that cannot be obtained from other nutrients 

 in the feed. 



The energy requirement of a fish may be satisfied by fat or protein, as 

 well as by carbohydrate. If sufficient energy nutrients are not available in 

 the feed the body will burn protein for energy at the expense of growth 

 and tissue repair. The use of carbohydrate for energy to save protein for 

 other purposes is known as the "protein- sparing effect" of carbohydrate. 



Carbohydrate energy in excess of the immediate energy need is convert- 

 ed into fat and deposited in various tissues as reserve energy for use during 

 periods of less abundant feed. Quantities in excess of needed levels lead to 

 an elevated deposition of glycogen in the liver, and eventually will cause 

 death in salmonids. 



Fat-infiltrated livers and kidneys in salmonids are a result of fat deposi- 

 tion within the organ, resulting in reduced efficiency and organ destruc- 

 tion. This condition results primarily from excess levels of carbohydrates in 

 the feed. 



Carbohydrates also may serve as precursors for the various metabolic in- 

 termediates, such as nonessential amino acids, necessary for growth. Thus, 

 in the absence of adequate dietary carbohydrates or fats, fish may make 

 inefficient use of dietary protein to meet their energy and other metabolic 

 needs. In addition to serving as an inexpensive source of energy, starches 

 improve the pelleting quality of fish feeds. 



Dietary fiber is not utilized by fish. Levels over 10% in salmonid feeds 

 and over 20% in catfish feeds reduce nutrient intake and impair the digesti- 

 bility of practical feeds. 



CARBOHYDRATES IN SALMONID FEEDS 



Carbohydrates are an inexpensive food source, and there is a temptation to 

 feed them at high levels. However, trout are incapable of handling high 

 dietary levels of carbohydrates. The evidence for this is the accumulation 

 of liver glycogen after relatively low levels of digestible carbohydrate are 

 fed. Trout apparently cannot excrete excessive dietary carbohydrate. In 

 higher animals, excessive carbohydrate is excreted in the urine. Such ex- 

 cretion does not occur in trout even though the blood sugar is greatly in- 



