Copper alloys are important netting materials in aquaculture (the farming of aquatic organisms including fish farming). Various other materials including nylon, polyester, polypropylene, polyethylene, plastic-coated welded wire, rubber, patented twine products (Spectra, Dyneema), and galvanized steel are also used for netting in aquaculture fish enclosures around the world.Offshore Aquaculture in the United States: Economic considerations, implications, and opportunities, U.S. Department of Commerce, National Oceanic & Atmospheric Administration, July 2008, p. 53Southern Regional Aquaculture Center at All of these materials are selected for a variety of reasons, including design feasibility, material strength, cost, and corrosion resistance.
What sets copper alloys apart from the other materials used in fish farming is that copper alloys are antimicrobial, that is, they destroy bacteria, viruses, fungi, algae, and other microbes. (For information about the antimicrobial properties of copper and its alloys, see Antimicrobial properties of copper and Antimicrobial copper alloy touch surfaces).
In the marine environment, the antimicrobial/algaecidal properties of copper alloys prevent biofouling, which can briefly be described as the undesirable accumulation, adhesion, and growth of microorganisms, plants, algae, tube worms, barnacles, mollusks, and other organisms on man-made marine structures.Marine Fouling and its Prevention, Wood Hole Oceanographic Institution, 1952, United States Naval Institute, Annapolis, Maryland, USA By inhibiting microbial growth, copper alloy aquaculture pens avoid the need for costly net changes that are necessary with other materials. The resistance of organism growth on copper alloy nets also provides a cleaner and healthier environment for farmed fish to grow and thrive.
In addition to their antifouling benefits, copper alloys have strong structural and corrosion-resistant properties in marine environments.
Due to these properties – antifouling, high strength, and corrosion resistance – copper alloys are considered a desirable material for marine applications such as condenser tubing, water intake screens, ship hulls, offshore structure, and sheathing. In the past 25 years or so, the copper alloys have become widely used in the marine aquaculture industry. The industry is now actively deploying copper alloy netting and structural materials in commercial large-scale fish farming operations around the world.
Growth of aquaculture
Aquaculture, an industry that has emerged only in recent decades, has become one of the fastest growing sectors of the world food economy. Aquaculture already supplies more than half of the world's demand for fish.Half of Fish Consumed Globally Is Now Raised on Farms, Study Finds Science Daily, 8 September 2009 This percentage is predicted to increase dramatically over the next few decades.
Biofouling
Biofouling is one of the biggest problems in aquaculture.Design Guide: Copper Alloy Mesh in Marine Aquaculture, International Copper Research Association Inc. (INCRA), 1984 Biofouling occurs on non-copper materials in the marine environment, including fish pen surfaces and nettings. For example, it was noted that the open area of a mesh immersed for only seven days in a Tasmanian aquaculture operation decreased by 37% as a result of biofouling.
The biofouling process begins when algae spores, marine invertebrate larvae, and other organic material adhere to surfaces submerged in marine environments (e.g., fish nets in aquaculture). Bacteria then encourage the attachment of secondary unwanted colonizers.
Biofouling has strong negative impacts on aquaculture operations. Water flow and dissolved oxygen are inhibited due to clogged nets in fish pens.Ahlgren, M.O., (1998), Consumption and assimilation of salmon net pen fouling debris by the red sea cucumber Parastichopus califormicus: Implications for poly-culture, Journal of the World Aquaculture Society, Vol. 29, pp. 133–139 The result is often diseased fish from infections, such as netpen liver disease, amoebic gill disease, and parasites. Other negative impacts include increased fish mortalities, decreased fish growth rates, premature fish harvesting, reduced fish product values and profitability, and an adversely impacted environment near fish farms.
Biofouling adds enormous weight to submerged fish netting. Two hundredfold increases in weight have been reported.Milne, P.H., (1970), Fish Farming: A guide to the design and construction of net enclosures, Marine Research, Vol. 1, pp. 1–31 This translates, for example, to two thousand pounds of unwanted organisms adhered to what was once a clean 10-pound fish pen net. In South Australia, biofouling weighing 6.5 tonnes (approximately 13,000 pounds) was observed on a fish pen net. This extra burden often results in net breakage and additional maintenance costs.
To combat parasites from biofouling in finfish aquaculture, treatment protocols such as cypermethrin, azamethiphos, and emamectin benzoate may be administered, but these have been found to have detrimental environmental effects, for example, in lobster operations.
To treat diseases in fish raised in biofouled nets, fish stocks are administered antibiotics. The antibiotics can have unwanted long-term health effects on consumers and on coastal environments near aquaculture operations.The next seafood frontier: The ocean, 28 April 2009, references article by Myers in Nature; To combat biofouling, operators often implement costly maintenance measures, such as frequent net changing, cleaning/removal of unwanted organisms from nets, net repairs, and chemical treatment including antimicrobial coatings on nylon nets.Li, S. (1994), Fish culture in cages and pens: Freshwater Fish Culture in China: Principles and Practice, pp. 305–346, Elsevier, Amsterdam The cost of antifouling a single salmon net can be several thousand British pounds. In some sectors of the European aquaculture industry, cleaning biofouled fish and shellfish pens can cost 5–20% of its market value. Heavy fouling can reduce the saleable product in nets by 60–90%.Collective research on Aquaculture Biofouling
Antifouling coatings are often used on nylon nets because the process is more economical than manual cleaning. When nylon nets are coated with antifouling compounds, the coatings repel biofouling for a period of time, usually between several weeks to several months. However, the nets eventually succumb to biofouling. Antifouling coatings containing cuprous oxide algaecide/biocide are the coatings technology used almost exclusively in the fish farming industry today. The treatments usually flake off within a few weeks to six to eight months.Alberto, Jose and Disselkoen, Ochoa (2009), Floating device to clean nets, Patent application 12/455,150, Publication US 2010/0006036 A1, Filing date 27 May; and National Chilean Patent Application No. 1565-2008 filed on 29 May 2008
Biofouled nets are replaced after several months of service, depending on environmental conditions, in a complicated, costly, and labor-intensive operation that involves divers and specialized personnel. During this process, live fish in nets must be transferred to clean pens, which causes undue stress and asphyxiation that results in some loss of fish.Paclibare et al., (1994), Clearing of the kidney-disease bacterium Renibacterium salmoninarum from seawater by the blue mussel Mytilus edulis, and the status of the mussel as a reservoir of the bacterium, Diseases of Aquatic Organisms, Vol. 18, pp. 129–133 Biofouled nets that can be reused are washed on land via manual brushing and scrubbing or high-pressure water hosing. They are then dried and re-impregnated with antifouling coatings.Beveridge, M. (2004), Cage Aquaculture. The University Press, Cambridge Enright, C., (1993), Control of fouling in bivalve aquaculture, World Aquaculture, Vol. 24, pp. 44–46Lee et al., (1985), Observations on the use of antifouling paint in netcage fish farming in Singapore, Singapore Journal of Primary Industries, Vol. 13, pp. 1–12
A line of net cleaners is available for in-situ washings where permitted.Idema Net Cleaning Systems But, even where not permitted by environmental, fisheries, maritime, and sanitary authorities, should the lack of dissolved oxygen in submerged pens create an emergency condition that endangers the health of fish, divers may be deployed with special in situ cleaning machinery to scrub biofouled nets.
The aquaculture industry is addressing the negative environmental impacts from its operations (see aquaculture issues). As the industry evolves, a cleaner, more sustainable aquaculture industry is expected to emerge, one that may increasingly rely on materials with anti-fouling, anti-corrosive, and strong structural properties, such as copper alloys.
Antifouling properties of copper alloys
In the aquaculture industry, sound animal husbandry translates to keeping fish clean, well fed, healthy, and not overcrowded.Offshore Aquaculture in the United States: Economic Considerations, Implications, & Opportunities, U.S. Department of Commerce, National Oceanic & Atmospheric Administration, July 2008 One solution to keeping farmed fish healthy is to contain them in antifouling copper alloy nets and structures.
Researchers have attributed copper's resistance to biofouling, even in temperate waters, to two possible mechanisms: 1) a retarding sequence of colonization through release of antimicrobial copper ions, thereby preventing the attachment of microbial layers to marine surfaces;Sutherland, I.W., 1983, Microbial exopolysaccarides: Their role in microbial adhesion in aqueous systems, Critical Reviews in Microbiology, Vol. 10, pp.173–201 and, 2) separating layers that contain corrosive products and the spores of juveniles or macro-encrusting organisms.Edding, Mario E., Flores, Hector, and Miranda, Claudio, (1995), Experimental Usage of Copper-Nickel Alloy Mesh in Mariculture. Part 1: Feasibility of usage in a temperate zone; Part 2: Demonstration of usage in a cold zone; Final report to the International Copper Association Ltd.
The most important requirement for optimum biofouling resistance is that the copper alloys should be freely exposed or electrically insulated from less noble alloys and from cathodic protection. Galvanic coupling to less noble alloys and cathodic protection prevent copper ion releases from surface films and therefore reduce biofouling resistance.Powell, Carol and Stillman, Hal (2009), Corrosion behavior of copper alloys used in marine aquaculture
As temperatures increase and water velocities decrease in marine waters, biofouling rates dramatically rise. However, copper's resistance to biofouling is observed even in temperate waters. Studies in La Herradura Bay, Coquimbo, Chile, where biofouling conditions are extreme, demonstrated that a copper alloy (90% copper, 10% nickel) avoided macro-encrusting organisms.
Corrosion behavior of copper alloys Copper alloys used in sea water service have low general corrosion rates but also have a high resistance to many localized forms of corrosion. A technical discussion regarding various types of corrosion, application considerations (e.g., depth of installations, effect of polluted waters, sea conditions), and the corrosion characteristics of several copper alloys used in aquaculture netting is available (i.e., copper-nickel, copper-zinc, and copper-silicon).
Early examples of copper sheathing