Comparing off‐flavor trends in freshwater recirculating aquaculture systems with microbially mature or immature biofilters while growing Atlantic salmon <scp> <i>Salmo salar</i> </scp>

Abstract Geosmin (GSM) and 2‐methylisoborneol (MIB) cause objectionable off‐flavors in fish produced in recirculating aquaculture systems (RAS). Remediation solutions have not been developed; therefore, a study was conducted to determine if microbial maturity limits off‐flavor production. Triplicate RAS with newly established nitrification (“immature”) were compared to “mature” RAS operated continuously for 2.5 years, while growing market‐size Atlantic salmon, Salmo salar . Mean waterborne GSM and MIB levels peaked at 35–40 ng/L in the immature RAS but were maintained at &lt;13 ng/L in the mature RAS. Similar trends were reflected in salmon flesh. After 2 months, fillet GSM levels in the immature and mature RAS were 696 ± 31 and 247 ± 30 ng/kg ( p = 0.001) respectively, and MIB was consistently higher in salmon from the immature RAS. The abundance of off‐flavor‐producing organisms was not associated with off‐flavor trends, suggesting production was related to the RAS environment. Total ammonia nitrogen, nitrite‐nitrogen, total suspended solids, heterotrophic bacteria count, and true color were significantly higher in the immature RAS, and nitrifier abundance was generally lower and less stable. Of these, machine learning identified true color as the most important feature affecting GSM. Ultimately, the microbially mature RAS minimized off‐flavor in water and salmon flesh.