Fish Hatchery Management - fisheries & aquaculture

NUTRI'I'ION AND FIEDING 233Other substances such as algal blooms, muskgrass, chemicals, and organiccompounds can produce undesirable flavors in fish. When the water temperatureis high, as it is in late summer, there is a greater chance that off-flavorswill occur in fish flesh.Organic Toxicants in FeedsNumerous naturally occurring and synthetic organic compounds producetoxic responses in fish. Tannic acid, aflatoxin, and cyclopropenoid fatty acidsall induce liver cancer in fish. Gossypol, a toxin present in untreated cottonseedmeal, causes anorexia and ceroid accumulation in the liver. Phyticacid, which ties up zinc in the feed, and growth inhibitors found in soybeanmeal can be destroyed by proper heating during processing. Chlorinatedhydrocarbons occur as contaminants of fish meal and can cause mortalitywhen present in fry feeds. Broodfish transfer these compounds from the feed totheir 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 toxicantsto maintain the health and efficient production of fish. Symptoms ofsome organic toxicants are given in Appendix F.Sources of FeedsNATURAL FOODSAs the name implies, natural foods are obtained from the immediate environment.Small fish feed upon algae and zooplankton. As the carnivorousfish grow, they devour progressively larger animals-insects, worms, mollusks,crustaceans, small fish, tadpoles, and frogs. Many fish remain herbivorousthroughout their lives.Pondfish culturists take advantage of the natural feeds present in stillwaters. The composition of insects, worms, and forage fish used as fish foodis mostly water (75-U0'li,). the remaining components are protein (tZ-tS'lr),fat (3-7%), ash (1-4'X,), and a little carbohydrate (less than 1%). Duringwarm weather when insects hatch and bottom organisms are abundant, apond can provide a considerable amount of feed for fish. This productioncan be increased by pond fertilization. Because the environment tends tobe highly variable in its production of biomass, natural methods of providingfood are inefficient unless the producer is utilizing large bodies of water.Natural food organisms are relied upon to provide nutrients lacking inthe supplemental feeds used in pond culture.