AQUACULTURE AND FERTILIZATION A training course in Chipata, Zambia. By Henk van der Mheen Fish,...


A training course in Chipata, Zambia.

By Henk van der Mheen
Fish, like all animals, need food to grow. The amount of food they consume determines how fast they grow. Consequently, food availability to a large extent determines what the production of a fish pond will be and how much income the farmer will receive. In most cases fish farming is not a hobby but an activity to generate income and food. It is, therefore, important to ensure fish get enough to eat.

Most smallholder farmers in Africa can not buy commercial fish feeds because these are either not available or expensive. Farmers as well as many extension workers are of the opinion that nutrient inputs for proper pond management are lacking in much of Southern Africa, and that this short-coming is an important reason for low pond production. The training course for the staff of the Department of Fisheries in Chipata, Zambia, addressed this issue and explained that a wide variety of inputs can be used to increase fish production in ponds.

Fish can be fed directly or indirectly. Direct feeding means that consumable inputs are provided to the fish. Indirect feeding means that inputs are provided that enter the food chain at some point and eventually end up in the fish. This indirect method is called fertilization.

The immediate mental link is between fertilization and fertilizers, especially chemical fertilizers in the form of small round pellets. These chemical fertilizers contain inorganic nutrients which can be used by plants. When fish ponds are fertilized with inorganic fertilizers the nutrients enter the food chain at the level of the phytoplankton. Some fish species eat phytoplankton, but before the nutrients reach any other fish species they have to go through various steps in the food chain. Every step means a loss in energy.

Since all organic materials contain nutrients, organic materials can be used for fertilization. Because the concentration of nutrients in many organic materials is less than those in chemical fertilizers, people often do not expect very good results when using organic fertilizers. This would be true if, as in land- based agriculture, the food chain has to start with plants. In fish farming this is not the case. Fish eat zooplankton, benthos and detritus. The quantity of zooplankton, benthos and detritus can directly be increased through fertilization with organic materials. With organic fertilization the fish production does not have to start at the bottom of the food chain but can start at a level closer to the fish, consequently with less energy loss.

Fertilizing with organic materials can, therefore, increase fish production considerably. Moreover, for most smallholder farmers it is easier and cheaper to get plant materials, like grasses, leaves etc., than it is to purchase chemical fertilizers.

To have a real effect on the fish production, it is necessary to apply enough organic material. For a 100 m2 pond, 15 kg of fresh plant material and 5 kg of manure should be applied weekly. This material should be put in a large compost enclosure and stirred regularly to ensure uniform decomposition and that it becomes available for production and is not compacted in a small crib where the fish can not have access to it.

All these issues were discussed during the Chipata training, and the effect of different fertilizers was demonstrated with plankton samples taken from different fish ponds with different levels of nutrient management. Ponds rich in primary nutrients had more phytoplankton and less benthos and zooplankton than ponds fertilized with grass.

In Chipata the fisheries staff will now use this information to set up demonstrations with selected farmers so other fish farmers can see the benefits of pond fertilization with plant material. They will also compile lists of locally available organic materials that can be used by farmers for pond fertilization.

In fact most plant materials can be used, and it is important farmers understand that, although prepared fish feeds are scarce, there are always inputs available that can be used to increase fish production.