Copper Oxide

Copper is biocidal and the oxide form is incorporated into treatments such as paints for aquaculture netting.

Methodology

Most finfish antifouling net coatings currently incorporate Copper Oxide (Cu2O) as the active ingredient. Nearly all commercial net coatings are water-borne (no organic solvents) and have Cu2O as the prime biocide in concentrations up to 20%, often in combination with one or more organic booster biocides such as SeaNine, Zinc pyrithione (=Zinc Omadine) and Dichlofluanid.
Nets after manufacture or after servicing are dipped in paint solution for 2-4 hours and dried by machine or air. Uptake of the coating treatment is approximately 1 litre of treatment per kg of net. Treatment can be diluted to allow better penetration into the netting particularly for previously treated nets. Pressure washing or underwater disk cleaning of copper coated netting is not recommended, except as an emergency measure, as the coating can be removed from the netting. (http://www.ukmarinesac.org.uk/activities/ports/ph4_3_1.htm#a1)

Key Factors Positive

- Efficacy at lowering the rate of fouling is good.
- At present, copper based net coatings are the only cost effective net treatment available.

Key Factors Negative

- Environmental effects.
- Biocidal coatings need to obtain approval from authorities such as the Biocidal Product Directive (BPD).
- Limited to a maximum of 1 season and not sufficiently effective against algae.
- Waste removal and waste water after cleaning must be processed responsibly.
- The red colour of copper paints was one of the worst performers (only in terms of colour) in experiments in the CRAB project. See the CRAB best Practice Guidelines, page 46 for more information.

Cost Benefit

For netting the current cost (2007) is approximately 4 Euro per litre paint/kg netting. Cost of treatment varies with location and is likely to increase with the increasing demand for copper as a raw material. Results from CRAB suggest that cleaning costs can in some cases be roughly halved for netting with a copper treatment. Periodic cleaning and re-treatment is required which is costly.

Conclusions/ Discussion

Due to its complex nature and the uncertainty over its level of interaction with other substances, it is difficult to establish the precise effect of elevated levels of copper in the marine environment. It is therefore difficult to generalise about the toxicity of copper to marine organisms. There is evidence that certain species of fish are sensitive to quite low levels of copper even though other species are tolerant of much higher levels. Benthic marine organisms are thought to be slightly more sensitive to copper than fish, although some species demonstrate a capacity to adapt to elevated levels. Legislative pressure on biocidal antifoulings is increasing but there are no clear indications that copper based products will be prohibited in the near future for aquaculture.