NUTRITION AND FKEDING 221 



for the fish, presence of toxic substances, and the quality of the lipid. Fish 

 and vegetable oils that are polyunsaturated are more easily digested by fish 

 than 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 deter- 

 mining the amount to be used in the feed for a given fish species. Lipids 

 are a primary source of energy for fish and have a protein- sparing effect. 

 Therefore high levels in the feed would be beneficial. However, high fat 

 levels in the feed can hamper the pelleting of feeds and cause rapid 

 spoilage of feed during storage. 



Rancidity of lipids, especially of polyunsaturated oils, due to oxidation 

 can be a problem in fish feeds. Rancid lipids have a disagreeable odor and 

 flavor and can be toxic to fish. The toxic effects may be due to products of 

 the oxidation of the lipid itself or to secondary factors such as destruction 

 of vitamins or mold growth. Oxidation of lipids in the feed often results in 

 the destruction of vitamins, especially vitamin E. The oxidation process 

 also produces conditions that favor mold growth and breakdown of other 

 nutrients. Because rancid lipids in the feed are detrimental to fish, every ef- 

 fort should be made to use only fresh oils protected with antioxidants. 

 Feeds should be stored in a cool, dry area to minimize oxidation of the 

 lipids in the feeds. 



Contamination of fish feeds, especially those for fry and broodstock, with 

 pesticides and other compounds such as polychlorinated biphenols (PCB) 

 cause many health problems and may be lethal in fish. Fish oil is a com- 

 mon source of contaminants in fish feeds. Because most contaminants are 

 fat-soluble they accumulate in the fatty tissues of fish. When fish oil is ex- 

 tracted from fish meal, these compounds are concentrated in the oil. Fish 

 used in the production of fish meal and oil pick up these compounds from 

 their natural foods in a contaminated environment. Feed manufacturers 

 should select only those fish oils that contain low levels or none of these 

 compounds. Vegetable oils, which are naturally free of these compounds, 

 also can be used. 



LIPID REQUIREMENTS FOR SALMONIDS 



When there is little or no fat in the feed, a trout forms its own fat from car- 

 bohydrates and proteins. The natural fat of a trout is unsaturated with a 

 low melting point. Practical-feed formulators use fish oil and vegetable oil 

 in trout feeds as the primary energy source. These oils are readily digested 

 by the trout and produce the desired soft body fat. Hard fats such as beef 

 tallow are not as readily digested because they are not emulsified easily, 

 especially in cold temperatures. Hard fats can coat other foods and reduce 

 their digestibility, thus lowering the performance of the feed. Very hard 

 fats may plug the intestines of small trout. 



