Put a structure in the ocean, and you attract a host of life wanting to settle on it. Aquaculture nets can become home to a host of sedentary species, like algae, molluscs, and hydroids.
Unfortunately as these organisms accumulate, they can reduce water and oxygen flow through the net, decrease waste removal, increase the risk of disease to the farmed animals, and even physically damage the nets.
Aquaculturalists wishing to control such ‘biofouling’ have a few choices – physically remove the organisms, replace the nets, or use antifoulants to reduce the amount of biofouling occurring in the first place.
Copper is a naturally antimicrobial metal, and a common ingredient in biofouling paints that can be applied to nylon nets. Copper can even be used as part of the net-material itself. These copper alloy meshes also offer another advantages – reducing escapism or predation from holes that can develop in more traditional nylon-mesh settings.
The downside to copper is that at high levels, it can be toxic to marine organisms, raising concerns about the use of copper alloy nets in the industry.
However, as Dr Ioanna Kalantzi (Hellenic Centre for Marine Research, Greece) and colleagues demonstrated in a recently published study, the effects of copper alloy nets may not be so different from copper-based antifouling paints.
The team placed two experimental cages with gilthead seabream (Sparus aurata) off the coast of Greece – one a conventional nylon net cage with a copper-based antifouling paint, and the other a copper-alloy (brass) net.
Whilst the cages remained in situ for a whole year, the seabream were loaded for until their rearing period had been completed - six months at a time, with individuals collected and assessed at the end of the two rearing periods.
Transplanted caged mussels were also placed near each of the experimental cages, and were collected and tested every three months, and seawater and sediment samples were collected before the cages went in the water, and throughout the experiment.
Throughout the experiment, the amount of copper that accumulated in the seabream didn’t differ significantly between the two cages, nor did it ever reach safely limits set by the Food and Agriculture Organization (FAO) for human consumption.
The mussels, however, showed a different pattern. During the first six months of the experiment, those near the copper-alloy cage showed higher levels of copper in their system than their nylon-caged counterparts, but by the end of the experiment, no significant difference in copper levels was apparent.
When comparing to a third set of cages mussels that were located at a ‘pristine reference site’, it does seem that the copper levels around the two cages are higher than would normally be found in the environment.
About four months into the experiments, the mussels located near the two experimental cages unexpectedly showed increases in copper. The scientists think that the culprits were working cages located near their experimental cages, which may have undergone repairs or been replaced. Unlike copper-alloy cages, the antifouling paint on nylon cages needs to be replaced every six to eight months to remain effective, temporarily increasing the amount of copper being leached.
Like the mussels, the seawater samples showed higher copper levels around the copper-alloy cage than the treated nylon cage during the first three to four months, but balanced out after around nine months. Importantly, at no point did the copper from either cage reach maximum legal limits set by the European Commission Directive (Discharges of Dangerous Substances) for marine water quality.
In addition, copper from the cages did not seem to accumulate in the sediment directly below the cages or further afield.
The study highlights that copper-alloy cages may be no worse for the environment than coating nylon cages with copper-based antifoulants, however the scientists suggest that longer studies are needed to make sure that there are no long-lasting impacts of copper alloy cages.
