Common off-flavor compounds, including geosmin, are produced by specific bacterial populations that persist within microbial biofilms in recirculation aquaculture systems (RAS). Waterborne off-flavors released by these bacteria rapidly bioaccumulate in the flesh of RAS-produced fish resulting in unpalatable fillets that are objectional to consumers. To effectively remediate off-flavor from market-ready products, most RAS facilities relocate fish from grow-out to separate depuration systems prior to harvest. Facility-specific standard operating procedures to remediate off-flavor from fish flesh are generally established; however, certain aspects of the depuration process have not been well defined and significant room remains for improvement and refinement of this process, particularly for market-size Atlantic salmon Salmo salar produced in RAS.
As such, two research trials were carried out evaluating the depuration kinetics of geosmin from Atlantic salmon (> 5 kg) originally cultured in a semi-commercial scale RAS. Study 1 evaluated the effects of water flushing rates and associated depuration system hydraulic retention time on the kinetics of geosmin remediation. Study 2 assessed the effects of water rotational velocity, related fish swimming speed, and dissolved oxygen concentrations (metrics that affect fish metabolism) on geosmin reduction kinetics. Each research trial was carried out using twelve replicate partial reuse aquaculture systems (PRAS) each consisting of a 5 m3 culture tank, a gas conditioning column, and a low-head oxygenator. One day prior to each study, 320 Atlantic salmon were relocated from the semi-commercial scale RAS and stocked in an 18 m3 production tank within a separate PRAS. The next day, salmon were exposed to a concentrated geosmin solution (Sigma Aldrich) to boost concentrations of this off-flavor in the flesh. Salmon harvested from onsite RAS generally have fillet geosmin levels that are below the detection limit of the human pallet; therefore, geosmin dosing was necessary for this research. Time series water samples were collected during the four-hour geosmin dosing period to evaluate off-flavor uptake by the fish. Immediately following geosmin dosing, 26 salmon were moved from the holding tank into each PRAS to begin the respective trials. Fish (fillet) and water samples were collected on Days 0, 3, 6, and 10 for subsequent solid phase micro-extraction analysis of geosmin concentrations. Geosmin levels in water and fish flesh will be compared between treatments using ANOVA analyses, and practical considerations for cost, energy, and time savings will be evaluated as these factors relate to the study results.
At the time of the abstract submission deadline, data collection and processing for these research trials was still underway. Complete results will be available at the conference.