NUTRITION AND FEEDING 211 



rate, measured as gain in wet body weight or gain in length, increases to a 

 maximum and then decreases as temperatures approach the upper lethal 

 limit. The best temperature for rapid, efficient growth is that at which ap- 

 petite is high and maintenance requirements (or the energy cost of living) 

 are low. 



For every 18°F increase in water temperature, there is a doubling of the 

 metabolic rate and, as a result, an increase in oxygen demand. At the same 

 time that oxygen demand is increasing at higher temperatures, the oxygen 

 carrying capacity of the water decreases. The metabolic rate of the fish in- 

 creases until the critical oxygen level is approached. Just below this point, 

 the metabolic rate decreases. 



Temperature is a very important factor in establishing the nutrient re- 

 quirements of fish. To deal with this problem, the National Research 

 Council (NRC) reports Standard Environmental Temperatures (SET) for 

 various species of fish. Suggested Standard Environmental Temperatures 

 are 50°F for salmon, 59°F for trout, and 85°F for channel catfish. At these 

 temperatures the metabolic rate for these fish is 100"o. Caloric needs 

 increase with rising water temperatures, resulting in an increase in the 

 fishes' appetite. The fish culturist must, therefore, adjust the feeding rate or 

 caloric content of the feed to provide proper energy levels for the various 

 water temperatures. Failure to make the adjustment will result in less than 

 optimal growth and feed wastage. 



SPECIES, BODY SIZE, AND AGE 



Within the ranges of their optimal water temperatures, the energy require- 

 ments of warmwater fish are greater than those of equally active coldwater 

 fish of the same size. At the same water temperature, coldwater fish con- 

 sume more oxygen than warmwater fish, indicating a higher metabolic rate 

 and greater energy need. Carnivorous fish have a higher metabolic rate 

 than herbivorous fish because of the greater proportion of protein and 

 minerals in their diet. Even though fish efficiently eliminate nitrogenous 

 wastes through the gills directly into the water, more energy is required for 

 the elimination of wastes from protein utilization than from fats and car- 

 bohydrates. Species that are less active have lower metabolic rates and 

 energy requirements for activities than more active ones. In general, the 

 energy requirements per unit weight are greater for smaller than for larger 

 fish. Fish never stop growing, but the growth rate slows as the fish becomes 

 older. The proportional increase in size is greatest in young fish. 



PHYSIOLOGICAL CHANGES 



Spawning, seasonal, and physiological changes affect the rate of metabo- 

 lism. Growth rate becomes complicated with the onset of sexual maturity. 



