NUIRITION AND FEEDING 225 



warm-blooded animals, but this is of less importance to fish because a fish's 

 body temperature corresponds to environmental water temperatures. Usual- 

 ly, the body temperature of a resting fish will be at or near the environ- 

 mental water temperature. Free energy is available for biological activity 

 and growth and is used for immediate energy and for formation of body tis- 

 sue or is stored as glycogen or fat. 



Fish adjust their feed intake according to their energy needs. An exces- 

 sively high energy level in a feed may restrict protein consumption and 

 subsequent growth. Except for the extremes, fish fed low-energy feeds are 

 able to gain weight at a rate comparable to those fed high-energy feeds by 

 increasing their feed intake. If a feed does not contain sufficient nonprotein 

 energy sources to meet the fish's energy requirements then the protein nor- 

 mally used for growth will have to be used for energy. Therefore, it is diffi- 

 cult to determine a specific energy or protein requirement without consid- 

 ering the relative level of one to the other. Absolute figures on optimum 

 energy requirements are difficult to state in fish nutrition because fish can 

 be maintained with little growth on a low-energy intake or be forced to 

 produce more weight by feeding them in excess. To maintain optimum 

 growth and the efficiency of a feeding program, the feeding level should be 

 adjusted if energy levels of the feeds vary significantly. The feeding level 

 should be increased for low-energy feeds and decreased for high-energy 

 feeds. Energy needs for maintenance increase with rising water tempera- 

 tures and decrease when temperatures are reduced, thus requiring changes 

 in the feeding rates. However, more energy is required to produce weight 

 gains of fish at lower temperatures than at high temperatures. 



Fish normally use about 70"" of the dietary energy for maintenance of 

 their biological systems and activity, leaving about 30"o available for 

 growth. Energy requirements for vital functions must be met before energy 

 is available for growth. A maintenance- type feeding program is designed to 

 supply the minimum energy and other essential nutrients for the vital func- 

 tions and activity, with no allowance for growth. Dietary efficiency or feed 

 conversion are terms used to designate the practical conversion of food to 

 fish flesh. In this concept of estimating gross energy requirements, the 

 amount of food (energy) required to produce a unit of weight gain is deter- 

 mined. In general, if the conversion of food to fish flesh is two or less, en- 

 ergy requirements are being met. This is because energy for biological 

 maintenance of fish must be supplied before energy is available for growth. 



ENERGY REQUIREMENTS FOR SALMONIDS 



Brook, brown, rainbow, and lake trout have similar energy requirements. 

 Between 1,700 and 1,800 available dietary kilocalories are required to pro- 

 duce a pound of trout, depending upon the feed being fed and conditions 

 under which the fish are reared. The amount of available calories from fish 

 feeds depends upon the digestibility of nutrients by the fish. 



