Copper Cage Trial at Cahora Bassa, Mozambique
The Mozambezi Tilapia Farm in Mozambique had a major problem with the bio-fouling of their fish cages. The high nutrient loading from Lake Kariba spills into the Cahora Bassa and was clogging up the nets where the fish are kept. It is estimated that the resultant bio-fouling in the Cahora Bassa is twice as much as in Lake Kariba.
Polyethylene and rigid HDPE netting have traditionally been used to build the fish cages, but both these materials are particularly vulnerable to intense bio-fouling in the first three meters of the water column, which reduces water flow through the netting resulting in low dissolved oxygen levels. These conditions severely affect fish growth, health and food conversion efficiency, with high stress levels in the fish leading to heavy mortalities.
This bio-fouling also necessitates the cleaning of the nets at least weekly, during which time the fish must be removed from the nets, which is a very labour intensive exercise.
Attacks on the nets by predators, such as otters and crocodiles, are also not uncommon and to prevent damage to the holding nets, additional predator nets have to be installed.
Throughout the world, fish have been secured in copper cages without the need for an additional predator net and, as copper is not vulnerable to bio-fouling, the fish are healthier and the mortality rate is much lower.
At the Mozambezi Tilapia Farm, the CDAA employed the services of Advance Africa to manage the deployment, monitoring and evaluation of the copper nets, which were supplied by Copalcor, a large non-ferrous metal manufacturer in Germiston, Johannesburg. Mozambezi, the local operator, supplied the fish stock and also assisted with the installation.