A study in the journal Aquaculture followed the production cycle of a recirculating aquaculture system (RAS) pikeperch operation in Denmark. Researchers explored the impacts of geosmin, an organic compound present in RAS water that negatively impacts the taste-profile and market value of fish. They proposed that by monitoring the geosmin dynamics in RAS operations, producers could prevent the compound from proliferating and negatively impacting the market value of the fish. They also hypothesised that any shifts in the microbial populations of the RAS would raise the concentrations of geosmin and encourage the compound to grow in the system.
The case study demonstrated that basic management practices, especially cleaning the RAS, significantly increased geosmin levels. The researchers suggested that producers avoid cleaning the biofilters and denitrification units as the pikeperch approached slaughter weight.
More fish are being produced in RAS every year. The practice allows fish to be raised efficiently – it lowers the risk of negative environmental impacts of production while using less water and providing a healthy environment for fish.
Though RAS is a positive environmental step in fish farming, managing the water filtration systems and microbial populations is an ongoing challenge for RAS producers. Geosmin is a key example of this challenge – it’s the organic compound that causes the fresh and earthy smell many of us detect after a rain shower. The molecule, which is produced by bacteria in RAS operations, gives fish a muddy taste and reduces its market value.
RAS producers can counteract the effects of geosmin through depuration, but this isn’t an attractive option. Depuration not only adds to production costs, but it makes the fish lose weight due to feed restrictions and delays harvest. Finding a method to keep geosmin concentrations below 10ng/L (consumers can detect it above this level) would improve the quality of RAS pikeperch and increase their market value.
Case study and results
The researchers began by measuring geosmin levels in the various parts of the RAS. This would indicate which part of the system contributed more to overall levels while also alerting researchers to fluctuations in the compound over time.
Since the researchers took samples from the start of the RAS operation, initial concentrations of geosmin were low. However, large bursts of geosmin were recorded after cleaning the filters and denitrification systems. For the main filters, the initial increase in geosmin was offset after the system was flushed out. This trend was not observed during denitrification: geosmin levels peaked and remained high after the system was flushed.
Though the researchers detected geosmin in all areas of the RAS, some units had higher concentrations of the compound than others. The denitrification unit was found to be the biggest source and contributor of geosmin in the RAS. The researchers observed that if the unit was cleaned, aerated, drained or washed out, high volumes of geosmin were released into the system. After cleaning, it was difficult to remove geosmin from the recirculating water.
Based on evidence from this case study, the grow-out tanks and drum filters didn’t contribute to the overall level of geosmin in the RAS. This finding can be contrasted to other RAS case studies with rainbow trout and salmon, where geosmin in grow-out tanks contributed to low flavour quality in the fish.
Since the researchers observed similar geosmin levels at the inlets and outlets of the biofilters, they concluded that they were geosmin-neutral. The only spike in the compound from this part of the RAS occurred when the system was cleaned.
When testing the water quality in the RAS, the researchers found a positive correlation between the ammonium and alkalinity levels and the production of geosmin – meaning that as ammonium or alkalinity increased, more geosmin was detected. Based on this observation, the researchers encouraged RAS producers to track these indicators since they were detected alongside geosmin spikes.
The researchers concluded that basic management practices, especially cleaning the RAS, significantly increased geosmin levels. In order to counteract this, the researchers recommend that RAS operators avoid or limit cleaning biofilters and denitrification units before harvest. This will keep geosmin levels low and maintain the flavour profile of the fish.
Producers should also divert flushing flows away from the RAS and replace the water in the system with clean makeup water. Adjusting the frequency of cleaning and backwashing the biofilters and denitrification units will prevent geosmin from carrying over from filter cleaning.
Given the positive correlation between ammonium and geosmin in RAS water, the researchers also recommend improving the nitrification rate in the system – this will help keep ammonium levels low.