214 FISH HATCHERY MANAGEMENT 



of protein in the feed resulting only in reduced growth, whereas the lack of 

 any one of several vitamins produces well described deficiency signs. Nu- 

 trients such as protein and vitamins should be present in feeds at levels to 

 meet minimum requirements, but not in an excess which might be wasted 

 or cause other health problems. 



The nutrients to be discussed in this chapter include (l) protein, (2) car- 

 bohydrates, (3) fats, (4) vitamins, and (5) minerals. 



Protein Requirements 



The primary objective of fish husbandry is to produce fish flesh that is 

 over 50% protein on a dry weight basis. Fish digest the protein in most na- 

 tural and commercial feeds into amino acids, which are then absorbed into 

 the blood and carried to the cells. 



Amino acids are used first to meet the requirements for formation of the 

 functional body proteins (hormones, enzymes, and products of respiration). 

 They are used next for tissue repair and growth. Those in excess of the 

 body requirements are metabolized for energy or converted to fat. 



Fish can synthesize some amino acids but usually not in sufficient quan- 

 tity to satisfy their total requirements. The amino acids synthesized are 

 formed from materials released during digestion and destruction of proteins 

 in the feed. Certain amino acids must be supplied in the feed due to the 

 inability of fish to synthesize them. Fish require the same ten essential a- 

 mino acids as higher animals: arginine; histidine; isoleucine; leucine; 

 lysine; methionine; phenylalanine; threonine; tryptophan; valine. Fish fed 

 feeds lacking dietary essential amino acids soon become inactive and lose 

 both appetite and weight. When the missing essential amino acids are 

 replaced in the diet, recovery of appetite and growth soon occurs. 



In fish feeds, fats and carbohydrates are the primary sources of energy, 

 but some protein is also utilized for energy. Fish are relatively efficient in 

 using protein for energy, deriving 3.9 of the 4.65 gross kilocalories per 

 gram from protein, for an 84'a) efficiency. Fish are able to use more protein 

 in their diet than is required for maximum growth because of their effi- 

 ciency in eliminating nitrogenous wastes through the gill tissues directly 

 into the water. Nutritionists must balance the protein and energy com- 

 ponents of the feed with the requirements of the fish. Protein is the most 

 expensive nutrient and only the optimal amount should be included for 

 maximum growth and economy; less expensive digestible fats and carbohy- 

 drates can supply energy and spare the protein for growth. 



Several factors determine the requirement for protein in fish feeds. 

 These include temperature, fish size, species, feeding rate, and energy con- 

 tent of the diet. Older fish have a lower protein requirement for maximum 



