NUTRITION AND FEEDING 235 



The manufacture of expanded or floating pellets requires higher tem- 

 peratures and pressures. Under these conditions, raw starch is quickly gela- 

 tinized. Bonds are formed within the gelatinized starch to give a durable, 

 water-stable pellet. The sudden release of pressure following extrusion al- 

 lows water vapor to expand and the ensuing entrapment of gas creates a 

 buoyant food particle. The additional cost of producing floating feeds must 

 be carefully compared to the advantages of using a floating feed. Many 

 catfish producers prefer the floating feeds because they can observe the fish 

 feeding. This aids in pond management and reduces feed wastage due to 

 overfeeding and loss of pellets that sink into the bottom muds. Recent 

 studies with catfish have shown that feeding 15°<) of the ration as floating 

 feed and 85% as sinking feed gives better feed utilization and is more 

 economical than feeding either alone. 



Although the extrusion of feeds may result in the destruction of certain 

 vitamins, amino acids, and fats, the lost materials can be replaced by spray- 

 coating the pellets before packaging. Color may also be added at this time. 



A moist, pelleted fish feed containing 30-35% water can be made with 

 special ingredients and equipment. No heat is required in pelleting moist 

 feeds. Mold inhibitors, hygroscopic chemicals, or refrigeration must be used 

 to protect moist feeds against spoilage. After extrusion the pellets are 

 quick- frozen and stored at — 14°F. If properly handled, the pellets will 

 remain separate without lumping. Moist pelleted feed spoils rapidly when 

 thawed and a major loss of vitamins will result within a few hours. 



Moist feeds cost more to manufacture, ship, and store than dry pelleted 

 feeds because they must be kept frozen. But they are beneficial in feeding 

 fish that do not accept dry formulated feeds. Fingerlings of some species 

 prefer the soft moist feeds because they are similar in texture to natural 

 feeds. Moist feeds have been used successfully as an intermediate stage in 

 converting fish from natural food to dry formula feeds. 



Salmon producers are the major users of moist feeds. The Oregon moist 

 pellet can be obtained at a competitive price from several commercial feed 

 companies in the northwest. 



Open- and Closed-Formulated Feeds 



There are open and closed formula feeds. An open- formula feed is one for 

 which the complete formula is disclosed. Generally, such feeds have been 

 developed by state or federal agencies or universities. An open- formula 

 feed has the following advantages. 



(l) The producer knows exactly what is in the feed, including the level 

 of vitamin supplementation. 



