Copper in aquaculture

In March this year, South Africa's Department of Trade and Industry (dti) launched a R800-million incentive programme for marine and freshwater fishing projects aimed at growing the country's fledgling aquaculture industry. The Aquaculture Development and Enhancement Programme, launched by Trade and Industry Minister Rob Davies, intends to stimulate investment in this relatively untapped sector.

But, this industry is not without its challenges. According to Copper Development Association Africa's (CDAA) centre director, Evert Swanepoel, the damage inflicted by predatory species can lead to both fish and economic losses. The principal predators include aquatic mammals, for example otters and seals, reptiles like crocodiles, invertebrates, such as crabs, and birds.

Predators may cause damage to livestock or farm facilities either directly, indirectly or both. Direct damage results when the fish or other cultured organism is killed or seriously maimed by the predator and is therefore lost from production. Indirect damage is highly variable, and includes: non-lethal wounding of fish; chronic stress with a consequent reduction in feeding efficiency or health; transfer of harmful disease-causing organisms; and sometimes even physical damage to the animal enclosure system leading to escapement.

"Often, the indirect damage caused by a predator can result in a greater economic loss than that caused by direct damage. For example, a crocodile that tears a hole in a net-cage and eats a few fish is a small loss compared to the pending escape of potentially large numbers of the remaining fish," says Swanepoel.

Materials are selected for a variety of reasons

Various materials including nylon, polyester, polypropylene, polyethylene, plastic-coated welded wire, rubber and galvanised steel are commonly used for netting in aquaculture fish enclosures around the world. "All of these materials are selected for a variety of reasons, including design feasibility, material strength, cost, and corrosion resistance," explains Swanepoel.

He continues: "Recently, copper alloys have become important netting materials in aquaculture. What sets copper alloys apart from the other materials used in fish farming is that they have strong structural properties. Attacks on nets by predators are not uncommon and to prevent damage to holding nets made from traditional materials, additional predator nets often have to be installed. Globally in fish farms where fish have been secured in copper cages, it has been found that there is no need for an additional predator net. Copper also demonstrates corrosion-resistant properties in marine environments, which has made it the material of choice for decades in traditional applications such as condenser tubing, water intake screens, ship hulls, offshore structure and sheathing.

Additionally, in the marine environment, the antimicrobial/algaecidal properties of copper alloys actually prevent biofouling, making them a viable alternative to chemical biocides.

"It is the combination of all of these properties - high strength, corrosion resistance and antifouling - that has made copper alloys a desirable material for marine applications, with the aquaculture industry now actively deploying copper alloy netting and structural materials in commercial large-scale fish farming operations around the world, and more recently on the African continent too," concludes Swanepoel.