Copper impregnation of nets

Publish date: January 9, 2009

Installations in the sea will always be subject to fouling by shellfish, algae, barnacles and hydroids. Impregnation is used to reduce this fouling on the actual net, but also has other functions such as making the net stiff so that it is kept extended in the water, prevent UV radiation from ruining the net and reducing/filling in the gaps between the filaments in the net so that these areas are not fouled. Vannebo et al. (2000) estimated that 80-90% of discharges take place through leaching into seawater, whereas 10-20% takes place from net cleaning facilities. Total copper discharges in Norway arising from net impregnation are about 200 tonnes per year. That is, about 1 gram of copper is discharged for every 2 kilograms of farmed salmon produced.

Leaching of copper from fish farm nets and discharges of copper from net cleaning facilities comprise aquaculture’s chief addition of environmental toxins to the marine environment. Studies show that the discharges from net cleaning facilities, which constitute 10-20% of total copper discharges from aquaculture, lead to considerable excess concentrations of copper in sediments and biological material in areas near the cleaning facilities. Experiments have shown that copper discharges from net cleaning facilities are bioavailable for some evertebrates and have a markedly adverse impact on planktonic algae (NIVA, 1996). On the basis of this knowledge, it was wise of the authorities to set a requirement for zero discharges from net cleaning facilities. This was adopted in the form of a regulation in 2002, with effect from 2006 for existing facilities.

The environmental effects of the leaching of copper from fish farm nets
On the basis of figures from the Norwegian Pollution Control Authority, leaching of copper from fish farm nets in the sea amounts to about 160 tonnes of copper per year. Although the environmental effects of these diffuse discharges are not thoroughly documented, in light on its extent, thorough analyses ought to be performed of the effects that copper from fish farms has on benthic fauna, phytoplankton and other organisms. Copper ions are released into the open water and sink to the bottom. Copper compounds in the nets are also taken up by macroalgae and fauna that grow on the actual nets and sink to the bottom with them. The Norwegian Pollution Authority defines copper as an environmental toxic on the basis of:

  • Several copper compounds are classified as very toxic to aquatic organisms. The compounds can cause long-term effects in the aquatic environment.
  • Copper can accumulate in organisms and is expected to affect growth and reproduction of some aquatic animals.

Norway’s goal is to reduce copper discharges to water considerably by 2010.

Harmful concentrations
Although relatively few studies have been done to test the effect of heavy metals on marine species in a satisfactory manner, there are some studies that test acute toxicity. On the basis of these, tolerance thresholds can be calculated, but this produces estimates only and is not a sure way of establishing exact knowledge.

Marino-Balsa et al. (2000) tested acute mortality for three economically important and widespread species: lobster, spider or toad crab (Atlantic lyre crab) and the common prawn when exposed to the metals mercury, cadmium and copper. LC50 values were calculated after 48 hours for lobster and 72 hours for the other two species. Lobster was the most sensitive species. Marino-Balsa et al. conclude that the maximum concentration without a biological effect is 0.5 micrograms per litre of seawater for copper and mercury and 0.3 micrograms for cadmium. Bond et al. (1999) observed anatomical abnormalities in the ova of spiral wrack right after fertilisation that interfere with the cell division and development of the zygote. The threshold value for the changes was 10.6 nM. If the concentration was below 10.6 nM, the changes were reversible. Low tolerance thresholds for copper are also known from groups of organisms such as tunicates (Bellas et al. 2001). Anderson & Kautsky (1996) found that salinity affects the tolerance threshold for copper pollution. The most wide-ranging study so far, Hall and Anderson (1999), calculated an average tolerance threshold from a sample of 65 species in very different categories of organisms and found a value for copper of 5.6 micrograms per litre. On the basis of available studies there is reason to believe that many species will be unaffected by copper concentrations under 5 micrograms per litre, whereas some species that are rather important economically such as lobster may be harmed at even lower copper concentrations, possibly down to 0.5 micrograms per litre.

Copper concentrations around fish farms and net cleaning facilities
Despite proven hazardous effects, Bellona has not been able to find data on copper concentrations in water near fish farms and net cleaning facilities. However, it is easier to measure copper concentrations in sediments, and in this area there are a number of studies. Wilken (2001) found copper concentrations of over 800 milligrams per kilogram of sediment under fish farms in Denmark. However, fish farm sites in Norway almost always have better water exchange, so that conditions are not comparable. Similar figures were obtained in Scotland: 725 milligrams per kilogram (Miller, 1998).

Technological developments
Technological developments in aquaculture are proceeding very rapidly, also in respect of measures to prevent fouling of nets. This is amplified by the fact that impregnating nets constitutes a considerable cost in fish farming, which thus has a strong incentive to develop better solutions. The environmental gains are hailed by fish farmers, which view pollution of the fish farm site as a potential threat to their own business. The implementation of policy instruments against copper impregnation by the authorities may reinforce and hasten the implementation of better solutions.

SINTEF has examined the costs of various fouling prevention strategies, both with and without the use of copper impregnated nets.

  Cost pr. kg of fish (NOK) Cost pr. kg of fish (NOK)
Farm Strategy incl. investments excl. investments
Steel Impregnated nets, drying, change to unimpregnated net 0,40 0,33
Steel Impregnatad nets until change of big fish net, then unimpregnated nets 0,72 0,48
Plastic Impregnated combi nets 0,50 0,44
Plastic Impregnated nets, washing with flushing disc. 0,27 0,22
Plastic Impregnated nets, washing with flushing disc 0,35 0,30
Steel Impregnated nets, washing with flushing disc 0,53 0,31
Steel Umimpregnated nets, modified Nor Mær system 0,32 0,19
Steel Unimpregnated nets, Enviro drum system 0,13 0,09


Source: Begroingshindrende strategier, SINTEF (T. Olafson, 2006)

According to the report SINTEF action against on farms constitute to around. 2 to 4 percent of the production cost of producing 1 kilo of salmon. Further they conclude that the use of non-impregnated nets will require increased investment in technology or increased labour costs. At the same time the farmer saves the cost of expensive anti-fouling impregnation. As the table shows a steel farm with non-impregnated solutions is very reasonable in relation to a number of strategies based on continued use of copper anti fouling.

Examples of alternatives to copper impregnations
Double net bags: Especially prominent as of today are various solutions based on frequent changing of the nets. Systems with double net bags, where one bag can be pulled up and wiped off while the other remains in the water, have become quite popular, not only because fish farmers want to reduce pollution, but because such investments have proven to be cost-saving. The Nor-Mær company delivers a system that it calls the "environmentfriendly system". According to the company’s website,, the system is based on the use of double, unimpregnated nets. These are installed so that one net will always be hanging to dry on special net pillars. When the net in the water begins to be fouled, it is changed quickly and efficiently with the net that is hanging to dry. Nor-mær has developed special net winches for this use, allowing two people to change a net in one to one-and-a-half hours. Another vendor that has specialised in such solutions is Rabben Mekaniske Verksted, which markets its product under the name "Enviro Drum". A video presentation of the technology is posted at With such systems, there is no longer any need for cleaning and impregnating the nets. However, it requires that the nets are changed before fouling "gets the upper hand". By the very fact that users have to avoid serious fouling before changing nets, this type of cage means that the water exchange in the cage is good all the time, which in turn is important for health, quality and feed consumption.

In most cases, a system of double net bags makes copper impregnation superfluous, because the fouling falls off the net bag after drying. The dried organic material that sinks to the bottom in this instance hardly constitutes an environmental problem, since it contains neither copper nor other toxins. There is also reason to highlight a positive side effect of this technology. Frequent net changes allow the fish farmer to do adequate inspections of the nets, which may thereby prevent escapes of farmed salmon, considered to be the industry’s most serious environmental problem. This assumes that mechanical solutions for changing nets do not put too heavy a strain on the nets. So far, however, such systems have some limitations in locations especially subject to bad weather, where simpler circular plastic structures dominate.

Flushing nets
Although high-pressure flushing of fish farm nets has long been in use, it is taxing. The challenge is to develop mechanical solutions that make it possible to flush without using divers to go underwater, which is very expensive.

Wrasse eat fouling
In addition to the positive effect that wrasse has on fighting salmon lice, they have yet another benefit. It turns out that the wrasse nibble away at fouling from the nets. In combination with the mechanical methods mentioned above, wrasse can make fish farming nets without copper impregnation a more competitive alternative.