Pond Culture of Tilapia (part 3)
Fertilization and manuring
The most appropriate mouthbrooding tilapia for culture can feed low on the food chain, on a diet of plankton and detritus. If the natural productivity of a pond is increased through fertilization or manuring, significant production of tilapia can be obtained without supplemental feeds. Although yields are not as high as those obtained with feed, fertilizers and animal manures can be used to reduce the quantity and expense of supplemental feeds. An increase in natural food has a much greater effect on tilapia production at densities less than 4,000/acre.
Inorganic fertilizers are used less often because of their expense, but a single large application of an inorganic fertilizer high in phosphorus is frequently made prior to stocking fish to create an algal bloom. Tilapia productivity is stimulated mainly by an increase in phosphorus and to a lesser extent by an increase in nitrogen. Phosphorus is effectively increased through the application of liquid polyphosphate (13-38-0) at a rate of 20 pounds/acre (2.4 galions/acre).
Manuring, which is widely used for food fish production overseas, has not been practiced in the U.S. because of public perception. Manuring may have application in the production of tilapia as a source of fish meal for animal feeds. The quality of manure as a fertilizer depends on several factors. Pig, chicken and duck manures increase fish production more than cow and sheep manure. Animals fed high quality feeds (grains) produce manure that is better as a fertilizer than those fed diets high in crude fiber. Fresh manure is better than dry manure. Finely-divided manures provide more surface area for the growth of microorganisms and produce better results than large clumps of manure.
Manure should be distributed evenly over the pond surface area. Large accumulations of manure on the pond bottom produce low oxygen conditions in the sediment that reduce microbial activity and sometimes result in the sudden release of toxic chemicals into the water column.
To maximize fish production, manure should be added daily to the pond in amounts that do not reduce dissolved oxygen (DO) to harmful levels as it decays. The maximum application rate varies from 90 to 180 pounds/acre/ day for dry manure. The maximum rate depends on the quality of the manure, the oxygen supply in the pond and water temperature. If early morning DO is less than 2 ppm, manuring should be reduced or stopped until DO increases. If it is not possible to measure DO, the maximum rate
should be limited to 90 pounds/acre/day to ensure a margin of safety. When water temperatures are less than 64° F, manuring should be discontinued. At low temperatures the rate of decomposition decreases and manure may accumulate on the pond bottom. A subsequent increase in temperature could then result in an oxygen depletion.
The rate of manuring should be increased gradually as the fish grow. The recommended manuring rate as dry matter is 2 to 4 percent of the standing fish biomass per day.
Yields of male monosex populations in manured ponds have been modest, but production costs are very low if the manure is free. For example, all-male hybrids (T. nilotica x T. hornorum, 29 grams) stocked at 4,000/acre will produce a net yield of 1,470 pounds/acre of 200-gram fish in 103 days when given fresh cattle
manure at an average rate (dry weight) of 46 pounds/acre/day. In comparison, fish receiving a commercial high-protein feed will give a net yield of 2,370 pounds/acre. Feeding costs per pound of production are two to twenty times higher for fish fed the commercial diet compared to fish receiving manure.
Collection, transport, storage and distribution of manure involve considerable expense and are major obstacles to manured systems. These problems can be overcome by locating the animal production unit adjacent to or over the fish pond so that fresh manure can easily be delivered to the pond on a continuous basis. Effective and safe manure loading rates are maintained by having the correct number of animals per pond surface area.
Maximum tilapia yields are obtained from the manure output of 2,000 to 2,200 chickens/acre, which deliver 90 to 100 pounds (dry weight) of manure/acre/day. Broiler flocks should be composed of three size groups to stabilize manure output. Several crops of chickens can be produced during a fish production cycle.
Approximately 24 to 28 pigs/acre are required to produce a suitable quantity of manure (90 to 100 pounds of dry matter/acre/day) for tilapia production. The pigs are usually grown from 44 to 220 pounds over a 6-month period.
Ducks are grown on ponds at a density of 300 to 600/acre. The ducks are generally raised in confinement, fed intensively, and allowed access to only a portion of the pond where they forage for natural foods and deposit their manure. Ducks that arc raised on ponds remain healthier than land-raised ducks. Also by raising ducks on ponds, feed wasted by the ducks is consumed directly by the fish. Since ducks reach marketable size in 10 to 11 weeks, staggered production cycles are needed to stabilize manure output.
Tilapia are best harvested by seining and draining the pond. A complete harvest is not possible by seining alone. Tilapia are adept at escaping a seine by jumping over or burrowing under it. Only 25 to 40 percent of a T. nilotica population can be captured per seine haul in small ponds. Other tilapia species, such as T. aurea, are even more difficult to capture. A l-inch mesh seine (with bag) of proper length and width is suitable for harvest.
Southern Regional Aquaculture Center, Publication No. 280, July, 1989
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