Effects of antifouling copper coating in aquaculture– implications for fish health, performance and sustainability of production

Biofouling, the growth of organisms and algae on fish pen nets, is a significant issue in fish farming. It can block water circulation, make disease management harder, and help spread bacterial diseases within the pens. To tackle biofouling, fish farmers have often coated the nets with copper-based materials. While this prevents biofouling, it also leads to the release of copper into the surrounding water. Copper released from these coatings is the largest source of environmental pollution linked to fish farming. In Norway alone, an estimated 1,400 metric tons of copper were released into the environment in 2017. The CAPS project aims to better understand how much copper is released into the water and how factors like environmental conditions and farm operations influence this release. Farmed fish and marine life in these areas are exposed to chronic copper levels, and understanding how copper affects their welfare is crucial. There is also industrial interest in this issue, as copper exposure can reduce fish growth and lower feed conversion efficiency. To study how waterborne copper affects Atlantic salmon, two trials were conducted in 2023: an acute exposure and a chronic exposure trial. The data collected from these trials, including feed intake, growth, and digestibility, has been sent to various labs for detailed analysis. Tissue samples will be examined for heavy metal content, and copper levels in the fish’s blood will be measured. Researchers are particularly interested in how copper impacts the gut-brain axis, stress, and metabolism. So far, all the fish brains have been dissected, and RNA has been extracted for gene expression analysis. Some gut samples have been selected for a more detailed analysis (RNA sequencing) to identify specific genes to target in future experiments. This RNA sequencing has been completed, but the data still needs to be analyzed. At the same time, work on the gut microbiota and tissue histology is also progressing. The gut microbiota is being analyzed using a specialized bioinformatics approach, which has proven a bit challenging but promises better results. Another analysis, involving short-chain fatty acids, is set to begin soon. This part of the project is being handled by a PhD student working between Norway and China. One challenge has been the limited amount of intestinal material, as the fish ate less than expected during the trials. Since both the microbiota and fatty acid analyses rely on this material, careful planning is required. Cortisol levels were originally planned to be measured as a stress indicator, but due to the limited amount of fecal and blood samples, researchers shifted focus to a broader metabolic panel, including glucose, electrolytes, and liver enzymes. These lab tests are finished, but the data is still being processed. To explore the sub-lethal effects of copper on Atlantic salmon, we have extracted RNA from the gills and liver from both trials. This RNA has been sequenced, and the resulting data is now being analyzed. We have also tested for oxidative stress markers and analyzed stress-related genes using qPCR Histological studies are underway, and copper levels in the tissue will soon be quantified. Initial findings show that even low levels of copper can affect stress and energy pathways in salmon, along with growth rates. During the acute exposure trial, significant toxic effects were observed in the gills, consistent with what is known about copper toxicity. In contrast, the liver showed weaker signs of stress. In the chronic exposure trial, the fish experienced cellular stress in both the liver and gills, and their growth was notably reduced. These results suggest that even copper levels as low as 20 µg/L can have serious sub-lethal effects on salmon. The project also studied the effects of copper exposure on tilapia. The results revealed that copper significantly reduced the growth of tilapia, weakened their intestinal barrier, lowered their antioxidant capacity and immune function, and caused inflammation in their hindgut. It also decreased the diversity of gut bacteria and altered the gut microbiota. However, when tilapia were given a probiotic supplement containing Bacillus velezensis T23, the fish showed improved liver health and antioxidant capacity. The supplement also helped strengthen their gut barrier and reduced inflammation. Moreover, it increased the abundance of beneficial bacteria such as Firmicutes and Lactobacillus. Some parts of the CAPS project are still in the planning stages. One upcoming trial is a germ-free fecal transplant study, and another will focus on cleaner fish. This latter trial, set for 2025, will explore how much copper accumulates in cleaner fish, which eat biofouling organisms on the nets, and whether this affects their growth and welfare.

To combat the issue of biofouling and biofilm formation, fish pen nettings are frequently coated in copper alloys. To date, there is very limited data on how this practice affects copper concentration in the water in and around fish pens. CAPS therefore aims to map the extent to which copper from antifouling coating is released into the surrounding environment and determine how this is affected by factors such as temperature, salinity, net-rising and duration since last coating. Moreover, while copper coating is an effective approach in preventing biofouling it also creates an environment in which farmed fish and the marine species in the area are subjected to chronic copper exposure. CAPS seeks to determine how chronic exposure to waterborne copper affects the health and welfare of farmed Atlantic salmon and cleaner fish. We hypothesize that copper exposure alters the microbiota composition, and that the altered signaling in the microbiota-gut-brain axis is an important factor underlying the impaired growth observed in fish exposed to waterborne copper. Therefore, we have set up the CAPS consortium to broadly investigate how waterborne copper affects the gut microbiota, the gut-brain signaling, stress, food-intake, growth rate, physiology, and overall metabolism. We will also explore the sub-lethal toxicological effects of copper exposure pertaining to energy metabolism in Atlantic salmon and investigate how cleaner fish, known to feed on crustaceans growing directly on the net, are affected by the practice of copper coating. In sum, this project will provide novel information regarding the impact of copper-coated fish nets on the sustainability and productivity of fish farming. As a result, the CAPS project is of both high environmental, policy and industrial interest.