Chemical Fertilization:
Mineral aqua cultural fertilizers are classified into two categories: nitrogen and phosphorus fertilizers. In aquaculture, potassium fertilizers are not considered, as their fertilizing impact seems generally non-existent.
Several nitrogen fertilizers are available on the market (urea, ammonium nitrate, ammonium sulfate, sodium nitrate, and calcium cyanamide).
Those containing urea or ammonia have an acidifying effect on the environment that can be counter balanced by the use of lime. The fertilizing contributions are more effective when they are frequently applied at a low dose rather than when they are applied at a low frequency but high amount. The generally recommended amounts lie between 5–10 kg N.ha- 1 every two weeks.
Various phosphorus fertilizers are available on the market. The spreading of a phosphorus fertilizer leads to phosphates release in water. The latter can precipitate when water is rich in calcium or if the pH is high.
Phosphates establish strong connections with the sediment, which makes them unavailable for the trophic net work. For this reason, it is generally advised to dissolve fertilizers before spreading. The generally recommended amounts are bet ween 8.75 to 17.5 kg P2O5.ha- 1 every two weeks.
The use of mineral fertilizers is sometimes limited because of the toxicity of some compounds. Nitrogen toxicity is high when nitrites or non-ionized ammonia are abundant. The toxic NH3 component of ammonia becomes increasingly abundant when the pH rises.
Spreading unbalanced fertilizers can also promote the development of cyanobacteria, which are frequently very harmful to fish farming (decreasing of growth, water de-oxygenation that can involve fish mortality). As some of these algae can use the atmospheric N2, it is generally considered that they develop when the N/P ratio is weak, but proliferations were observed in an opposite situation.
In fact, these algae seem to proliferate every time the environment is unbalanced (unbalanced N/P, ponds rich in iron etc).
Organic Fertilization:
Just like in terrestrial agriculture, organic manures can also be used to fertilize the ponds. They generally consist of agricultural wastes and/or by-products, so that they are generally inexpensive. They effectively stimulate the ecosystem, as they do not only contribute to the nitrogen and phosphorus enrichment, but also to that of carbon and organic matter.
That way, they do not only stimulate the autotrophic food chains, but also the heterotrophic ones. As a consequence, organic fertilizers allow reaching fish productions higher than those exclusively obtained with mineral manures. Table 11 gives the composition of some manure used in fish culture, but there may be considerable deviations from the values presented.
Table 11. Chemical composition of some organic manure. (from Delincé G. (1992) The Ecology Of The Fishpond Ecosystem, With Special Reference To Africa, 230 pp. Dordrecht, Netherlands : Kluwer Academic).
Trials were conducted to assess the respective effects of manures (layer chicken litter and dairy cow manure) and chemical fertilizers (urea + triple super-phosphate) on the production of Nile tilapia in earthen ponds. The results show that the better yields are obtained by using chicken manure (11.7 kg.ha- 1.day-1) instead of chemical fertilizer or cow manure (8.6 kg.ha-1.day-1).
The superiority of some organic fertilizers is confirmed by other experiments. In China, comparing chicken litter, pig manure and cow manure, the highest fish yields were obtained with pig and chicken manure (5.2 and 5.1 times the unfertilized control respectively), and the lowest with cow manure (3.9 times the unfertilized control).
On the other hand, their mineral content to dry matter ratio is 20 to 30 times weaker than that of inorganic fertilizers. Thus, 1 ton of organic manure is equivalent to approximately 45 kg of a 10:5:10 fertilizer.
A simple way to fertilize fishponds consists in breeding other animals over or near the pond. The problem is then to determine the livestock necessary to fertilize the pond. Table 12 shows some types of fish-animal associations and the productions that were obtained. The size of the associated breeding varies according to the level of intensification, to the weight and to the age of animals. For pigs, livestock generally recommended are 30 to 85 pigs.ha- 1 and for ducks, 1000 to 3500 ducks.ha- 1 . The main constraint is related to the fact that the farmers have to master perfectly the two breeding.
Table 12. Some fish-animal associations and related fish production (t.ha- 1.year-1).
The dissolved oxygen concentration results from a balance bet ween the phytoplankton autotrophic activity (main source of oxygen) and the bacterial heterotrophic one (main oxygen consumer). There is a typical daily evolution, characterized by a progressive oxygenation during the day (oxygen production by photosynthesis > consumption by bacterial activity) and a de-oxygenation during the night (oxygen consumption only, no photosynthesis).
As already seen, organic fertilizations stimulate the heterotrophic, oxygen consuming, activity. When in excess, they can involve total de-oxygenation and mortality of all aquatic organisms. For that reason, the maximum recommended spreading quantities are 100 to 150 kg of dry matter per hectare and day. More precisely, the recommended daily contributions (expressed as a percentage of fish biomass) are 3 to 4% for the bovine and liquid pig manure, 2 to 4% for the chicken litter, and 2 to 3% for the duck litter.
Some ponds do not ans wer correctly to fertilization because of their low pH. Indeed, acid mud strongly absorbs phosphate. Moreover benthic organisms, in particular bacteria, do not develop correctly when pH is too low and phytoplankton lacks carbon and calcium in low hardness and low alkaline water. To increase the benefit of fertilization, pH of mud must be comprised bet ween 6 and 7 and total alkalinity must be at least 20 mg.l-1 of CaCO3.
Lime spreading makes it possible to solve these problems, and they have other advantages. Thus, they make it possible to sterilize the pond during drying by eliminating the parasites and pathogens; they improve the decomposition of the organic matter; they accelerate the nitrification; they reduce the sediment redox potential; they allow the suspended or dissolved organic matter to flocculate and sediment and thus improve the light penetration in water. Lime can also be brought to compensate the acidifying effect of some nitrogen fertilizers. In this case, the necessary amounts are proportional to the quantity of fertilizer spread.
They are generally comprised between 85 and 170 kg for 100 kg of acidifying fertilizer. Lime must be spread on the bottom in drained ponds or at the water surface in full ponds. Agricultural lime spread quantities are generally comprised bet ween 2000 and 10000 kg.ha-1 and the operation must be repeated every 3 to 5 years. It is recommended to bring each year a half of the initial contribution.
Authors:
Lionel Dabbadie and Jerome Lazard
