Fish Hatchery Management - fisheries & aquaculture

N U'I'RITION AND FEEDING22rfor the fish, presence of toxic substances, and the quality of the lipid. Fishand vegetable oils that are polyunsaturated are more easily digested by fishthan saturated fats such as beef tallow, especially at colder temperatures.The optimal level of dietary lipid for fish feeds has not been established.Protein content of the feed, and type of fat need to be considered in determiningthe amount to be used in the feed for a given fish species. Lipidsare a primary source of energy for fish and have a protein-sparing effect.Therefore high levels in the feed would be beneficial. However, high fatlevels in the feed can hamper the pelleting of feeds and cause rapidspoilage of feed during storage.Rancidity of lipids, especially of polyunsaturated oils, due to oxidationcan be a problem in fish feeds. Rancid lipids have a disagreeable odor andflavor and can be toxic to fish. The toxic effects may be due to products ofthe oxidation of the lipid itself or to secondary factors such as destructionof vitamins or mold growth. Oxidation of lipids in the feed often results inthe destruction of vitamins, especially vitamin E. The oxidation processalso produces conditions that favor mold growth and breakdown of othernutrients. Because rancid lipids in the feed are detrimental to fish, every effortshould be made to use only fresh oils protected with antioxidants.Feeds should be stored in a cool, drv area to minimize oxidation of thelipids in the feeds.Contamination of fish feeds, especially those for fry and broodstock, withpesticides and other compounds such as polychlorinated biphenols (PCB)cause many health problems and may be lethal in fish. Fish oil is a commonsource of contaminants in fish feeds. Because most contaminants arefat-soluble they accumulate in the fatty tissues of fish. When fish oil is extractedfrom fish meal, these compounds are concentrated in the oil. Fishused in the production of fish meal and oil pick up these compounds fromtheir natural foods in a contaminated environment. Feed manufacturersshould select only those fish oils that contain low levels or none of thesecompounds. Vegetable oils, which are naturally free of these compounds,also can be used.LIPID REqUIRHMEN'I'S FOR SAT,MONIDSWhen there is little or no fat in the feed, a trout forms its own fat from carbohydratesand proteins. The natural fat of a trout is unsaturated with alow melting point. Practical-feed formulators use fish oil and vegetable oilin trout feeds as the primary energy source. These oils are readily digestedby the trout and produce the desired soft body fat. Hard fats such as beeftallow are not as readily digested because they are not emulsified easily,especially in cold temperatures. Hard fats can coat other foods and reducetheir digestibility, thus lowering the performance of the feed. Very hardfats may plug the intestines of small trout.