4. COMMON HAZARDS IN AFRICAN MAGUR CULTURE

4.1 Oxygen deficiency

There is no negative effect of low oxygen level on African magur. But in magur dominated polyculture system the other species may be endangered by oxygen depletion.

In some ponds high concentration of detritus matter can be accumulated because of various reasons such as over manuring, dover feeding, etc. These organic matters cause high plankton bloom. Usually enough oxygen is produced by photosynthesis at day time to balance the demand; but during the night, the oxygen reserves in the water are depleted and the fish may die of anoxia before sunrise. The following precautionary measures should be taken to avoid fish kill by oxygen depletion.

• Biological control with proper combination of polyculture detailed in previous chapters. The surface feeders keep the plankton growth under control.

• Continuous use of “horra” (bottom raker) during production cycle in those highly detritus rich ponds.

• Controlled use of supplementary feed.

• Controlled use of organic manures.

4.2 Organic overloading

Especially with monoculture of African magur huge amount of organic material is added into the pond in the form of unused feed and excreta. In traditional ponds, such as undrainable ponds, without the facilities of water exchange, organic materials decompose and thus deteriorates the water quality.

Over feeding is a usual management mistake in winter. It increases the accumulation of decaying materials at the bottom of a pond. Algal blooming and oxygen depletion occurs frequently in these ponds which can cause a real danger for carp species. The polluted water also causes bacterial diseases in African magur.

Organic pollution in a magur monoculture pond can be minimized by taking the steps mentioned below :

• Arrangement of water exchange system above 3 fish/m2 stocking rate is highly recommended.

• Dewatering, liming and sun drying the pond bottom after each production cycle.

• Removal of accumulated detritus from the pond bottom every 1 or 2 years. This detritus is an excellent fertilizer for horticulture.

• Regular checking on feed consumption by African magur. Daily ration of feed has to be decreased it feed is found not eaten.

• Feed should be mixed well so that the ingredients remain stick together for 10–15 minutes in the water.

• Supplementary manuring should be stopped until water quality improves.

4.3 Cannibalism

African magur fingerlings, when hungry, are cannibalistic in nature. It can not be completely prevented but it can be decreased. With growth, from above 50–100 g, cannibalism stops when adequately fed. The following measures are suggested to minimize the loss due to cannibalism.

• Stocking equal size fingerlings into the pond.

• Feeding the fish 2–3 times a day according to the schedule.

• Feeding the fish in at least 4 feeding areas of the pond.

• Separating the fastest growing from the slowest growing fingerlings by sampling.

4.4 Poaching from ponds

In many places, poaching of fish has been found as a major constraint to fish culture. In such cases, arrangements for guarding ponds at night should be made. Other precautionary measures recommended are --

• putting bamboo twigs or branches of trees in the pond water to block the effective use of nets by the poacher.

• fixing barbed wire at the pond surface in criss-cross manner is also fairly effective, particularly in small ponds.

• fencing the farm/fish pond.

Predation

Snakes, lizards and birds have been found to feed on magur fingerlings. Local methods traditionally used to scare off these predators should be practised. Following are some of the measures recommended --

• Shooting the predatory birds in and around the pond.

• Place poison injected eggs on the embankment;

• Destroy the nests of fish eating birds around the pond;

• Collect the eggs several from the poultry houses;

• The bridge from the pond dyke to the poultry house over the pond water should be removed at night.

4.5 Common diseases of African magur

In well managed culture conditions African magur is found to be more or less resistant to fish pathogens viz. virus, bacteria and parasites. Poor water quality, infected feed and rough handling can make the fish weaker and make them susceptible to diseases.

Stressed or infected fish can be easily recognized by some abnormal behaviors, such as poor appetite and abnormal swimming behaviour, (staying in vertical position on the pond surface). In addition, some clinical symptoms such as mutilated barbels or fins, white or red-brown spots on the skin, pop-eyes etc could be observed. The health conditions of magur should be monitored daily, particularly during feeding when magur frequently come to the surface water. For diagnosing bacterial, fungal and parasitic diseases, squash preparations of the skin, the gill filaments, intestines etc. have to be made and examined.

Specific curative measures should be taken as early as possible. Some common African magur diseases and their curative measures as reported by Viveen et al. (1986) are discussed below:

a. Bacterial infections

Columnaris disease

Hemorrhagic septicaemia

Fungal disease
Saprolegniasis

Diseases due to malnutrition and environmental stress
Pop-eye, crackhead and spinal deformity :