NUTRITION AND FEEDING 219 



creased. In trout, the accumulation of blood glucose follows the same pat- 

 tern as that in diabetic humans. 



No absolute carbohydrate requirements have been established for fish. 

 Trout nutritionists have placed maximum digestible carbohydrate values 

 for feeds at 12-20"o. Digestible carbohydrate values are determined by 

 multiplying the total amount of carbohydrate in the feed by the digestibil- 

 ity of the carbohydrates. Digestibility values of various carbohydrates are: 

 simple sugars, 100%; complex sugars, 90%; cooked starch, 60%; raw starch, 

 30%; fiber, 0%. 



Digestible carbohydrate levels over 20''o in trout feeds will cause an ac- 

 cumulation of glycogen in liver, a fatty infiltrated liver, fatty infiltrated 

 kidneys, and excess fat deposition, all of which are detrimental to the 

 health of the fish. 



Levels of carbohydrates up to 20"ii can be tolerated in trout feeds in 

 55-65°F water. These same feeds fed in water below 50°F will cause exces- 

 sive storage of glycogen in the liver and can result in death. Carbohydrates 

 should, therefore, be limited in trout feeds. However, there are definite 

 beneficial effects from the carbohydrate portion of the feed. It can supply 

 up to 20"i) of the available calories in a feed, thus sparing the protein. The 

 energy from carbohydrates available to mammals is 4 kilocalories per gram, 

 whereas the value for trout is only 1.6 kcal/g, a 40%i relative efficiency. 



Most trout feeds do not contain excessive amounts of digestible carbohy- 

 drate. A balance between plant and animal components in the feeds gen- 

 erally will assure a satisfactory level of digestible carbohydrate. The major 

 sources of carbohydrate in trout feeds are plant foodstuffs, including soy- 

 bean oil meal, cereal grains, flour by-products, and cottonseed meal. Most 

 animal concentrates such as meat meals, fish meals, tankage, and blood 

 meals, are low in carbohydrate (less than 1.0%). The high percentages of 

 milk sugar in dried skim milk, dried buttermilk, and dried whey may cause 

 an increase in blood sugar and an accumulation of glycogen in the liver if 

 fed at levels greater than lO"" of the feed. 



Pacific salmon have been reported to tolerate total dietary carbohydrate 

 levels as high as 48"(i, with no losses or liver pathology. The digestible car- 

 bohydrate value would be lower, depending on the forms of the carbohydrate. 



CARBOHYDRATES IN CATFISH FEEDS 



Dietary carbohydrates are utilized by catfish, but only limited information 

 is available on their digestibility and metabolism. Channel catfish utilize 

 starches for growth more readily than sugars. In feeds containing adequate 

 protein, fish weight increases with the level of starch, but remains essential- 

 ly the same regardless of the amounts of sugar in the feed. Liver abnormal- 

 ities, poor growth, and high mortality observed in salmonids due to high 

 levels of dietary carbohydrates have not been found in catfish. 



