NUTRITION AND FEEDING 231 



Trace minerals are just as essential as major minerals, but are needed 

 only in small amounts. The nine essential trace minerals are iron, copper, 

 iodine, manganese, cobalt, zinc, molybdenum, selenium, and fluorine. 



Mineral elements, both major and trace, are interrelated and balance 

 each other in their nutritional and physiological effects. The minerals that 

 form the hard and supporting structures of a fish's body (bone and teeth) 

 are principally calcium and phosphorus. Very small amounts of fluorine 

 and magnesium also are essential for the formation of bones and teeth. For 

 normal respiration iron, copper, and cobalt are required in the red cell and 

 deficiencies of any of these trace elements may cause anemia. Sodium, 

 chlorine, and potassium play an important role in regulating body 

 processes and osmotic pressure. Minerals also are required for reproduc- 

 tion. They are removed from the female system during egg production and 

 must be replenished by adequate amounts in the feed. 



Most researchers agree that fish require all of the major and trace ele- 

 ments. Under normal conditions, chloride ions are exchanged very rapidly 

 from both food and water. Calcium and cobalt are absorbed efficiently 

 from the water but are utilized poorly from feeds. The level of calcium in 

 the water influences the uptake of the calcium from the food, and vice 

 versa. 



Feeds are a major source of phosphorus and sulfur. Inorganic phosphorus 

 is absorbed efficiently from the stomach and intestine of trout. The skin 

 (including the scales) in trout is a significant storehouse for calcium and 

 phosphorus. 



Only one mineral deficiency is recognized definitely in trout; as in 

 higher animals, a deficiency of iodine causes goiter. The study of the 

 mineral requirements of fish is incomplete, but it is apparent that both dis- 

 solved and dietary minerals are important to the health and vigor of fish. 



Nonnutritive Factors 



Although nonnutritive factors do not contribute directly to the mainte- 

 nance, growth, or reproduction of fish, they should be considered in the 

 formulation of rations as they can affect feed efficiency and the quality of 

 the final marketable product. Three nonnutritive factors — fiber, pigment- 

 producing factors, and antioxidants — warrant discussion concerning fish 

 nutrition. 



FIBER 



Due the simple structure of the gastrointestinal tract of fish, the digestibil- 

 ity of fiber in fish is extremely low, less than 10"o. Very little microbial 



