Biofilters in recirculating aquaculture systems (RAS) may play a more important role in fish health than previously understood, according to new research published in the journal Aquaculture.
Researchers found that biofilters do not simply remove ammonia from the water. Instead, they continuously release bacteria into the rearing environment, helping to shape the microbial communities to which fish are exposed.
The study involved three controlled experiments using Atlantic salmon fry in laboratory-scale RAS. Researchers operated pairs of identical systems that differed only in the biofilters they contained. The biofilters originated from separate commercial RAS facilities and harboured distinct microbial communities.
When the researchers exchanged the biofilters between systems, the microbial composition of the rearing water shifted to more closely resemble that of the newly introduced biofilters. Bacterial variants associated with the replacement biofilters also appeared in the rearing water.
The findings provide evidence that biofilters directly influence rearing water microbiota through the release of biofilm-associated bacteria.
According to the authors, this means bacteria residing within biofilters may interact directly with fish and could therefore affect fish health and performance.
The researchers said the findings have practical implications for the management of land-based aquaculture systems. Mature biofilters are expected to favour slow-growing, competitive bacterial communities known as K-strategists, while suppressing fast-growing opportunistic bacteria that can include pathogens.
Conversely, the presence of undesirable bacteria within biofilters could present risks to fish growth, health and survival.
The authors concluded that maintaining stable and mature biofilter communities may be important for preserving microbial water quality and optimising fish performance in RAS facilities.
The study was led by Sujan Khadka and co-authored by Fernando Fernando, Simen Fredriksen, Anna Aasen, Nicholas Dembe, Olav Vadstein and Ingrid Bakke.