NUTRITION AND FEEDING 213 



their food, but gulp it down intact. Pharyngeal teeth are used by some 

 species to grind feed. 



The gizzard serves as a grinding mechanism in some species of fish. 



The stomach is for feed storage and preliminary digestion of protein. Very 

 little absorption occurs in the stomach. 



The finger-like pyloric ceca at the junction of the stomach and small intes- 

 tines are a primary source of digestive juices. 



The small intestine is the major site of digestion and receives the digestive 

 juices secreted by the liver, pancreas, pyloric ceca, and intestinal walls. 

 The absorption of the nutrients occurs in this area. 



Some water absorption occurs in the large intestine, but its primary function 

 is to serve as a reservoir of undigested materials before expulsion as feces. 



Oxygen and Water Requirements 



Oxygen and water normally are not considered as nutrients, but they are 

 the most important components in the life-supporting processes. 



All vital processes require energy, which is obtained from the oxidation 

 of various chemicals in the body. The utilization of oxygen and resulting 

 production of carbon dioxide by the tissues is the principal mechanism for 

 the liberation of energy. Oxygen consumption by a fish is altered by size, 

 feed, stress, water temperature, and activity. The oxygen requirement per 

 unit of weight decreases as fish size increases. High- nutrient feeds, density, 

 stress, elevated water temperatures, and increased activity all increase oxy- 

 gen requirements of fish. As a consequence, adequate oxygen must be sup- 

 plied to assure efficient utilization of the feed and optimal growth. 



Water is involved in many reactions in animal systems either as a reac- 

 tant or end product. Seventy- five percent of the gain in weight during fish 

 growth is water. Water that is not provided in the feed itself must be taken 

 from the environment. Because water always diffuses from the area of 

 weakest ionic concentration to the strongest, water readily diffuses through 

 the gills and digestive tract into freshwater fish. In saltwater fish, the blood 

 ion concentration is weaker than that of marine water, so that the fish loses 

 water to the environment. This forces the fish to drink the water and ex- 

 crete the minerals in order to fulfill their requirements. 



A nutritionally balanced feed must contain the required nutrients in the 

 proper proportion. If a single essential nutrient is deficient, it will affect 

 the efficient utilization of the other nutrients. In severe cases, nutrient defi- 

 ciencies can develop, affecting different physiological systems and produc- 

 ing a variety of deficiency signs (Appendix F). Because all essential nu- 

 trients are required to maintain the health of fish, there is no logic to rank- 

 ing them in terms of importance. However, deficiencies of certain nutrients 

 have more severe effects than of others. This is exemplified by a low level 



