Fish Hatchery Management - fisheries & aquaculture

N T'TRIl-I()N AND FEEDING 235The manufacture of expanded or floating pellets requires higher temperaturesand pressures. Under these conditions, raw starch is quickly gelatinized.Bonds are formed within the gelatinized starch to give a durable,water-stable pellet. The sudden release of pressure following extrusion allowswater vapor to expand and the ensuing entraPment of gas creates abuoyant food particle. The additional cost of producing floating feeds mustbe carefully compared to the advantages of using a floating feed. Manycatfish producers prefer the floating feeds because they can observe the fishfeeding. This aids in pond management and reduces feed wastage due tooverfeeding and loss of pellets that sink into the bottom muds. Recentstudies with catfish have shown that feeding 15([l of the ration as floatingfeed and 85% as sinking feed gives better feed utilization and is moreeconomical than feeding either alone.Although the extrusion of feeds may result in the destruction of certainvitamins, amino acids, and fats, the lost materials can be rePlaced by spraycoatingthe pellets before packaging. Color may also be added at this time.A moist, pelleted fish feed containing 30-35% water can be made withspecial ingredients and equipment. No heat is required in pelleting moistfeeds. Mold inhibitors, hygroscopic chemicals, or refrigeration must be usedto protect moist feeds against spoilage. After extrusion the pellets arequick-frozen and stored at -14'F. If properly handled, the pellets willremain separate without lumping. Moist pelleted feed spoils rapidly whenthawed and a major loss of vitamins will result within a few hours.Moist feeds cost more to manufacture, ship, and store than dry pelletedfeeds because they must be kept frozen. But they are beneficial in feedingfish that do not accept dry formulated feeds. Fingerlings of some speciesprefer the soft moist feeds because they are similar in texture to naturalfeeds. Moist feeds have been used successfully as an intermediate stage inconverting fish from natural food to dry formula feeds.Salmon producers are the major users of moist feeds. The Oregon moistpellet can be obtained at a competitive price from several commercial feedcompanies in the northwest.Open- and Closed-Formulated FeedsThere are open and closed formula feeds. An open-formula feed is one forwhich the complete formula is disclosed. Generally, such feeds have beendeveloped by state or federal agencies or universities. An open-formulafeed has the following advantages.(t) fne producer knows exactly what is in the feed, including the levelof vitamin supplementation.