Managing the microbiome in a RAS farm

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Managing the microbiome provided a solution to off-fl avour compounds in Kuterra’s RAS system

One of the challenges facing recircula� ng aquaculture systems (RAS) – especially when these are used for grow-out – is the presence of off -fl avour compounds (OFCs) that can aff ect the smell and taste of the fi sh.

Geosmin is a typical example. It is a by-product of bacteria in the system and, while harmless, it is a smell to which we are very sensi� ve. O� en described as “the smell of the soil” or the aroma of woodland a� er a rainy day, humans can detect as li� le as 100 parts in a trillion of geosmin – and it is not a fl avour anyone wants in their fi llet of salmon.

Geosmin (and another OFC, “MIB” or 2-Methylisoborneol) are created when non-benefi cial microbes – o� en cyanobacteria or ac� nomyces – become established in RAS systems. These microbes produce Geosmin and MIB, which builds up in the recirculated water and is then taken up in the fl esh of fi sh in the system.

Off -fl avour is commonly treated by “purging” the fi sh using large volumes of new, clean water, known as makeup water (MUW) for between fi ve to 13 days before being harvested.

Fish are not fed during this purge and therefore lose weight (reducing their value). The large volumes of water required also increase the environmental footprint of each fi sh produced.

Raising land-based salmon on Vancouver Island in Bri� sh Columbia since 2013, Kuterra (now operated by Whole Oceans) produces high quality, sustainably reared fi sh. As one of the longest established grow-out facili� es, Kuterra has collated extensive OFC baseline data related to its systems. Typical for grow-out systems, Kuterra has historically managed OFC levels by

Above: Salmon Left: Full grown salmon Opposite from top: Cyanobacteria; Kuterra salmon; young smolt; Kuterra’s RAS facility

varying the levels of makeup water (MUW) in the system, that is, addi� onal water. However, OFC solu� ons must also improve water quality, ensuring MUW does not have to increase.

Having read about promising small scale trial work on OFC reduc� on conducted at the Aquaculture Centre of Excellence, Lethbridge College using ACF-RAS Right bacteria products, Kuterra contacted Nova Q and agreed to a full-scale trial at their facility.

Nova Q is an Irish fi rm of environmental sustainability specialists, off ering products and process innova� ons to customers across the agriculture, water, renewables, and aquaculture sectors.

RAS Right is an established brand of bacteria products developed by Nova Q and BluePlanet Labs for RAS Aquaculture. The range includes nitrifi ca� on, denitrifi ca� on, species that remove OFCs, and solu� ons for sludge reduc� on in freshwater and marine systems.

Nova Q applies the concept of “ac� ve microbiome management”, which involves the RAS Right bacteria species being mul� plied con� nuously on-site in a bioreactor grow-tank and dosed into the RAS biofi lter at regular intervals.

The current standard prac� ce is to only “seed” a biofi lter with benefi cial bacteria and allow bacteria popula� ons to develop without control. Simple seeding, however, does not control undesirable bacteria species taking hold in the biofi lter.

Ac� ve microbiome management ensures that not only are the OFCs targeted, but the essen� al popula� ons of nitrifying bacteria are maintained in adequate numbers to ensure water quality parameters are always met. The RAS Right bacteria species break down off fl avour compounds directly and “crowd out” the bacteria that produce the OFCs.

“Maintenance dosing” involves con� nually dosing RAS biofi lters with specifi cally selected bacteria formula� ons. This stops sub-op� mal combina� ons of bacteria species developing, ensuring robust nitrifi ca� on, and avoiding spikes in toxins such as H₂S or contaminants like geosmin.

Within a month, geosmin levels started a sustained fall. A� er 115 days, geosmin stabilised below the 5ng/L (fi ve parts per trillion) industry-standard limit for acceptable fl avour. This reduc� on was achieved while reducing MUW requirements to a quarter of their ini� al levels, alongside a downward trend in turbidity levels. Strong nitrifi ca� on – the core func� on of the biofi lter bacteria – was maintained throughout.

John Burton, Produc� on Manager at Kuterra, said: “We’re now ge� ng geosmin levels consistently below our maximum limit, coupled with only requiring design rates of makeup water – which is unprecedented in our site.”

Julian Bea� y, MD at Nova Q, said: “Kuterra has been an ideal site on which to prove out our geosmin solu� on at a larger scale. The team there are among the most experienced in the grow-out industry, both in how they monitor and in willingness to try new approaches in their RAS. These ground-breaking results show how OFC issues can be solved by ac� ve microbiome management”. FF

“We’re now ge� ng geosmin levels consistently below our maximum limit”