Introduction
The total food fish consumption has increased globally over the past decades. Over the same period, aquaculture has become the main aquatic food producer as capture fishery production is decreasing due to overfishing and water pollution. Aquaculture provides food for a growing population and is a major source of essential nutrients. Despite these advantages, traditional intensive outdoor aquaculture systems can have serious negative environmental effects as for example eutrophication, the spread of disease or the release of chemicals. Therefore, new aquaculture production technologies are needed. Land-based recirculating aquaculture (RAS) fish and seafood production is a possible alternative. Potential advantages of RAS fish production are, for example, controlled breeding conditions, optimized water quality and disease control or lower environmental impact. RAS as new technology has several limitations and disadvantages which require further research and development. These challenges include high capital and production costs, high energy consumption, the necessity for continuous performance monitoring, difficulties in treating diseases, or a need for a high level of technological and biological know-how due to the complexity of the system and its interactions. One major challenge in RAS production is the accumulation of off-flavour (taste and odour causing compounds) in fish filets. The main objective of the study was to reduce geosmin concentrations in RAS rearing water for Atlantic salmon (Salmo salar) production and, by this, also reduce the accumulation of geosmin into the fish fillet.
Methods
A commercial scale freshwater RAS for Atlantic salmon production was monitored over several months in high resolution with GC-MS for geosmin in water before and after the installation of a protein skimmer (with ozone) in combination with a UVC system. Water parameters of the RAS and flow through purge system were monitored over the whole measurement period.
Results
Before the use of ozone treatment, together with protein skimming and UVC, geosmin concentrations fluctuated in a daily resolution between around 5 ng/l and 180 ng/l. A by-monthly fluctuation is visible, which correlates with biomass transfers within the farm. After the start-up of the additional treatment loop and a last peak of geosmin, the average geosmin concentration dropped but still fluctuated and stabilized at a low level between <1 ng/l and 11 ng/l (see Figure 1). These low levels of geosmin in process water results in a low depot of geosmin for fish to be purged, reducing the needed purge time, weight loss and water consumption.
Conclusion
Technical approaches to control the growth of geosmin-producing microorganisms and improve the removal of geosmin can reduce geosmin concentration in grow-out freshwater RAS with a high effect.