NUTRITION AND FEEDING 215 



growth than young fish do. Species vary considerably in their require- 

 ments; for example, young catfish need less gross protein than salmonids. 

 The protein requirements of fish also increase with a rise in temperature. 

 For optimal growth and feed efficiency, there should be a balance between 

 the protein and energy content of the feed. The feeding rate determines the 

 daily amount of a feed received by the fish. When levels above normal are 

 fed, the protein level can be reduced, and when they are below normal it 

 should be increased to assure that fish receive the proper daily amount of 

 protein. Fish culturists can reduce feed costs if they know the exact pro- 

 tein requirements of their fish. 



The quality, or amino acid content, is the most important factor in op- 

 timizing utilization of dietary proteins. If a feed is grossly deficient in any 

 of the ten essential amino acids, poor growth and increased feed conver- 

 sions will result, despite a high total protein level in the feed. The dietary 

 protein that most closely approximates the amino acid requirements of the 

 fish has the highest protein quality value. Animal protein sources are gen- 

 erally of higher quality than plant sources, but animal proteins cost more. 

 Vegetable proteins do not contain an adequate level of certain amino acids 

 to meet fish requirements. Synthetic free amino acids can be added to feed, 

 but there is still some question as to how well fish utilize them. Thus, ami- 

 no acid balance at reasonable cost is best achieved by using a combination 

 of animal proteins, particularly fish meal, and vegetable proteins. 



Fish meal seems to be the one absolutely essential feed item. Most of the 

 ingredients of standard catfish feed formulas can be substituted for, but 

 whenever fish meal has been left out poorer growth and food conversion 

 have resulted. 



Fish cannot utilize nonprotein nitrogen sources. Such nonprotein nitro- 

 gen sources as urea and di-ammonium citrate, which even many non- 

 ruminant animals can utilize to a limited extent, have no value as a feed 

 source for fish. They can be toxic if present in significant levels. 



The chemical composition of fish tissue can be altered significantly by 

 the levels and components of ingredients in feeds. Within limits, there is a 

 general increase in the percentage of protein in the carcass in relation to 

 the amount in the feed. Furthermore, there is a direct relation between the 

 percentage of protein and that of water in the fish body. A reduction of 

 body protein content in fish is correlated with increased body fat; fish fed 

 lower- protein feeds have more fat and less protein. 



PROTEIN IN SALMONID FEEDS 



The protein and amino acid requirements for salmon and trout are similar. 

 The total protein requirements are highest in initially feeding fry and de- 

 crease as fish size increases. To grow at the maximum rate, fry must have a 



