NUTRITION AND FEEDING 233 



Other substances such as algal blooms, muskgrass, chemicals, and organic 

 compounds can produce undesirable flavors in fish. When the water tempera- 

 ture is high, as it is in late summer, there is a greater chance that off-flavors 

 will occur in fish flesh. 



Organic Toxicants in Feeds 



Numerous naturally occurring and synthetic organic compounds produce 

 toxic responses in fish. Tannic acid, aflatoxin, and cyclopropenoid fatty acids 

 all induce liver cancer in fish. Gossypol, a toxin present in untreated cot- 

 tonseed meal, causes anorexia and ceroid accumulation in the liver. Phytic 

 acid, which ties up zinc in the feed, and growth inhibitors found in soybean 

 meal can be destroyed by proper heating during processing. Chlorinated 

 hydrocarbons occur as contaminants of fish meal and can cause mortality 

 when present in fry feeds. Broodfish transfer these compounds from the feed to 

 their eggs, resulting in low hatchability and high mortality of swim- up fry. 

 Toxaphene affects the utilization of vitamin C in catfish and can cause the 

 "broken back syndrome." The environment and feed should be free of toxi- 

 cants to maintain the health and efficient production of fish. Symptoms of 

 some organic toxicants are given in Appendix F. 



Sources of Feeds 



NATURAL FOODS 



As the name implies, natural foods are obtained from the immediate en- 

 vironment. Small fish feed upon algae and zooplankton. As the carnivorous 

 fish grow, they devour progressively larger animals — insects, worms, mol- 

 lusks, crustaceans, small fish, tadpoles, and frogs. Many fish remain her- 

 bivorous throughout their lives. 



Pondfish culturists take advantage of the natural feeds present in still 

 waters. The composition of insects, worms, and forage fish used as fish food 

 is mostly water (75-80"o). The remaining components are protein (l2-15%), 

 fat (3-7%), ash (l-4"(i), and a little carbohydrate (less than 1%). During 

 warm weather when insects hatch and bottom organisms are abundant, a 

 pond can provide a considerable amount of feed for fish. This production 

 can be increased by pond fertilization. Because the environment tends to 

 be highly variable in its production of biomass, natural methods of provid- 

 ing food are inefficient unless the producer is utilizing large bodies of wa- 

 ter. Natural food organisms are relied upon to provide nutrients lacking in 

 the supplemental feeds used in pond culture. 



