Fish in a Tank- Aquaculture at Serepta Farm, Harare

By Andre Hoffman

Tank culture of Tilapia is a better alternative to a pond or cage culture if there is sufficient water and limited land is available, as the economics are more favorable. The advantage of intensive tank culture over pond culture is that the former can produce very high yields on small parcels of land and small tank volumes make it practical and economical to treat diseases with therapeutic chemicals dissolved in the culture water. The disadvantage is the limited access to natural foods in tanks, the fish must be artificially fed, and the cost of pumping water and aeration increases production costs.

About 7 months ago Serepta embarked on a journey to transforming our fish project by redesigning its system and changing feed from commercial supplements to Black Soldier Fly (BSF).  The principal goal and motivation was to better the returns.

 The system – The redesigning of the farm’s practices from the makeshift system to a more concentrated number of fish in a tank took some determination and perseverance;  to get the system going initially a number of hiccups were encountered before finally mastering the art.  The system currently holds 2000 Tilapia in a 7500 liter tank. The tank used is an Intex swimming pool. The high holding capacity of the number of fish in the tank is archived and maintained by an oxygenation technique of pumping oxygen throughout the waterbody. As a cost reduction strategy, Serepta developed the Venturi Aeration system, where atmospheric air is pumped into the water.

Feed – The system change was coupled with the production and adoption of the BSF as a substitute for commercial feed. Fish growth averaged 50g per week before feeding with the BSF, and  growth on average  increased to 80g/week. This growth rate is dependent on environmental temperatures, when temperatures are low fish grow slower. The farm is embracing vermicomposting, a technique for growing worms to feed the fish as an added supplement to the BSF maggots.  

Water Management – Tilapia grow well at high densities in the confinement of tanks when good water quality is maintained. Therefore, the culture systems employed is called a flow-through system, which involves discarding water after use. 20-30% of the water is emptied every 3 days to reduce the risks of ammonia toxicity. This translates to about 2000 to 2500 litres/day of ‘wastewater’. Drain design is thus critical for tank culture to facilitate the discarding of the water.

Dual Usage of Water -Fish farming is compatible with crop irrigation and can be integrated to increase the profitability of both production systems. The water is initially used for fish production and then for crop irrigation. Considering the scarcity of water resources in Zimbabwe, the wastewater drained from the fish tanks is used to irrigate the horticulture section at Serepta Farm. The waste water is highly fertilized, thus reducing fertilizer costs. The crops in the fields are as a result healthier due to the high concentration of oxygen in the water.  

A study indicated that basil yield was 79% greater if irrigated with fish effluent and nitrogen uptake 7-fold greater in maize as compared to field irrigation with unoxygenated fertilizer.

Better Returns – Serepta is not only experiencing exponential growth in fish but we are also getting better results in horticulture. The fish have become a by-product of the fertilizer/horticulture system. Fish become ready for sale in say 8 to 9 months of crop irrigation. What a bonus.

Backlinks – For more information on Vermicomposting and Black Soldier Fly (BSF) please refer to ZiMunda Farming Newsletter issue 5 and 6, respectively.

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