Biofouling is a challenge for salmon aquaculture worldwide. The unwanted growth of marine organisms such as mussels, algae, and hydroids on nets and other structures can impact hydrodynamics in and around the cage, increase risk of diseases, and may lead to behavioural changes in cleaner fish (used as biological salmon lice control). Biofouling is commonly controlled by a combination of antifouling coatings on nets and in-situ high-pressure cleaning of nets. However, high-pressure net cleaning can have a negative impact on fish health and welfare and will abrade net coatings, reducing the coatings efficacy. To support the sustainable growth of salmon aquaculture, novel net cleaning technology is needed.
The aim of the NOTVASK project was to develop new knowledge, technology and operational methods for the cleaning of biofouled nets. Novel cleaning technology should improve the cleaning efficacy while reducing the cleaning frequency, abrasion of the coating, energy and time consumption, and the release of particles.
The project was conducted by a consortium consisting of Brynsløkken AS, PSO AS, Lerow AS, Mowi ASA, Sperre AS, and SINTEF Ocean.
Data collected in a survey among 51 site managers provided a good overview of the biofouling situation in Norway, indicating similar biofouling related challenges for most sites. The main driver for biofouling management was the use of cleaner fish. Furthermore, many sites reported that net cleaning does impact the fish and that feeding regimes are often adjusted in response, confirming the need to improve cleaning practices.
As potential alternative technologies for net cleaning, low-pressure, cavitation-, and suction-based cleaning were assessed in field tests and compared to high-pressure cleaning. Cavitation cleaning reduced abrasion of antifouling coating to a maximum of 10% - a nine-fold improvement over high-pressure cleaning, while having similar (and at times higher) efficacy as high-pressure cleaning. In comparison, low-pressure cleaning had a significantly lower cleaning efficacy yet still a considerable impact on coating integrity. Suction cleaning was not able to remove sufficient biofouling from the test samples. Thus, this study identifies cavitation cleaning as promising technology for biofouling control on aquaculture nets that has the potential to provide efficient net cleaning while considerably reducing damage to antifouling coatings and subsequent contamination of the environment.
The feasibility of cavitation cleaning as an alternative to high-pressure cleaning was assessed in a cost-benefit analysis, taking into account the performance results from the field tests as well as information regarding energy consumption, technical requirements and potential impacts on the fish and other organisms in the surrounding area. The results indicate that cavitation cleaning has the potential to reduce energy consumption by 50% while offering similar cleaning performance as high-pressure cleaners. Since reduced abrasion will promote the coating's longevity and likely reduce cleaning frequency, cavitation cleaning could make net cleaning more sustainable. Further assessments of this technology, including tests of larger prototypes, should be conducted.
Based on the collected information, the project identified three future biofouling management strategies.
Strategy 1 is based on an efficient biocidal antifouling product that eliminates the need for all net cleaning. Thus, negative impacts on fish health and potential environmental pollution associated with the release of cleaning waste are prevented. A coating for this strategy should be a highly efficient yet environmentally benign biocide that allows good leaching control.
Strategy 2 acknowledges the difficulty of developing an omnipotent biocide and therefore combines a good antifouling product with intermittent net cleaning. By relying on a robust antifouling material combined with gentle net cleaning, the abrasion of the antifouling surface can be reduced, keeping its functionality intact and enabling long intervals between cleaning events. Ideally, cleaning waste should be collected to further prevent impacts on the fish and the environment.
Strategy 3 is based on regular grooming of the net without the use of antifouling. By cleaning the net at an interval shorter than the growth cycle of biofouling organisms, the build-up of a mature biofouling community is prevented, and the released cleaning waste should be limited to harmless particles. Using nets with protective coatings that facilitate biofouling removal may further support the success of this strategy.
While these strategies are partly applicable today, a focussed research and development effort is needed to implement the strategies with their individual benefits to their full potential.
This collaboration between researchers, the farming industry and associated service companies enabled a deep understanding of biofouling impacts on salmon farming. By collecting new knowledge on current biofouling management practices and their feasibility, and showing the value of such data, the project provides incentives and examples for the adoption of similar practices to the industry. The identification of cavitation as a promising new net cleaning technology enables service providers to develop the technology further and bring novel cleaning equipment to the aquaculture market. By proposing new control strategies, the project facilitates the development and use of novel methods and technologies. This information facilitates fish farmers, service providers, and coating and cleaning equipment manufacturers to improve sustainability of salmon farming.
Biofouling, i.e. undesired growth of organisms on submerged surfaces is in several aspects a problem for aquaculture nets. One of the challenges is related the use of cleaner fishes to keep the salmon lice count low. The cleaner fish preferably feeds on the biofouling and consequently the effect of cleaner fish against lice is reduced. Most fish farming companies use a combined strategy for biofouling management based on the use of copper coated nets and high pressure net cleaning. Unfortunately, todayís net cleaning technology is not sufficiently effective. One problem with high pressure cleaning is connected to the removal of copper coating from the net and its release into the environment, which is not meeting the demands of standards like from the Aquaculture Stewardship Council (ASC). Additionally, there are indications that some of the removed biofouling may harm fish gills.
Brynsløkken AS has hence the aim to develop new types of coating and cleaning technologies together with a consortium. The main focus of the innovation is to evaluate and develop 4 different net cleaning concepts: 1) vacuum cleaning that sucks biofouling of the net, 2) low pressure cleaning which may be used in higher frequency or on modified net surfaces and materials, 3) cleaning with a cavitationblaster that utilizes energy from the collapse of cavitation bubbles for removal, 4) induction cleaning based on net heating.
These selected technologies have the potential to clean:
- without damaging coatings
- more efficient so that quick re-growing of fouling organisms is prevented and consequently cleaning frequencies can be reduced
- without releasing biofouling wastes which may harm the fish
- more time and energy efficient
- in a way that fulfills the demand of the ASC
The project includes furthermore studies for the industrialization of the net cleaning concepts and the development of a best practice guide for net cleaning.