Aquacultures now supply around half of all edible fish worldwide – despite all the problems associated with it. A new kind of coating for the nets makes a contribution to making the net cultures more environmentally friendly. In the future, it should prevent their overgrowth without releasing harmful chemicals into the sea. First long-term tests of the new coating have led to promising results.
Worldwide, more and more freshwater and marine fish are being grown in ponds, breeding tanks and net enclosures. According to the UN World Food Program, every second edible fish worldwide comes from such aquacultures. Although this relieves the free fish stocks, it creates new problems. The droppings and feed residues of the often relatively crowded farmed fish also fertilize the surrounding water areas. If they are treated with antibiotics, this is also distributed in the water and soil and promotes resistance to bacteria.
What to do about network growth?
Another problem: micro- and macroorganisms such as algae, mussels or barnacles quickly settle on the nets that limit the farmed fish. “This growth is persistent and makes the nets heavier. This could tear them and we could lose our fish stock, ”explains marine biologist and fish breeder Yvonne Rößner from the Christian-Albrechts-University Kiel (CAU). “In addition, the overgrown meshes reduce the exchange of nutrients, which jeopardizes the health of the fish.” Typically, the nets of aquacultures are therefore coated with copper-containing paints that prevent a strong settlement of algae, mussels or barnacles. However, these coats release harmful substances into the water.
Rößner and her colleague Sophie Bodenstein want to prevent this. So far, they have been pulling the nets out of the water every few days and letting them dry by the wind and sun. “This is time-consuming and exhausting manual work, but this means that we can completely do without the chemical treatment of our net enclosures,” says Rößner. In order to find a better and less complex solution, the two fish farmers participated in a biofouling research project at the University of Kiel. The scientists draw on their experience with an environmentally friendly paint system for ships and examine which strategies can be adopted and adapted for networks.
Polymer composite as “Teflon” for nets
A promising candidate is a coating made of a polymer composite with specially shaped ceramic particles. It ensures an extremely smooth surface to which organisms have difficulty adhering. Together with the network manufacturer Walter Kremmin, the scientists under the direction of Martina Baum from the CAU tested how different compositions of their polymer composite work on commercially available networks and how they can be applied. For long-term tests, they coated two types of nets with different material compositions and hung them at different depths in the Kiel Fjord in the fish farm at Rößner and Bodenstein.
The first results of the long-term tests with around 400 samples show that some coatings help against growth. “The organisms on our coated test nets can be wiped off very easily by hand,” explains Baum’s colleague Haoyi Qiu. “But it also depends on the thickness of our coating. If we apply too much, the nets will become too heavy and could tear. ”The researchers are already researching the optimum thickness and developing the coating further. The team has already registered a patent for the coating. The ultimate goal is to optimize them so that the water flow is sufficient to immediately carry away microorganisms and larvae so that they cannot even settle on the nets.
Source: Christian Albrechts University of Kiel