Depuration system flushing rate affects geosmin removal from market-size Atlantic salmon Salmo salar

Common off-flavor compounds including geosmin (GSM) bioaccumulate in fish cultured in recirculating aquaculture systems (RAS) resulting in unpalatable fillets that are objectional to consumers. Most RAS facilities relocate fish from grow-out tanks to separate depuration systems with increased water flushing to remediate pre-harvest off-flavors, but certain aspects of this procedure have not been optimized including characterization of water exchange rates that effectively diminish off-flavor. To this end, a study was carried out to evaluate the effects of flushing rate and associated depuration system hydraulic retention time (HRT) on GSM removal from Atlantic salmon Salmo salar originally produced in a semi-commercial scale freshwater RAS. Twenty-six fish (5?7 kg each) were stocked into twelve replicate depuration systems operated with system HRTs of 2.4, 4.6, and 11.3-h, respectively (N = 4). Geosmin was assessed at intervals in system water and fish flesh over a 10-day feed withholding period. Waterborne GSM concentration was affected by flushing rate and associated system HRT (P < 0.05). Depuration systems operated with an 11.3-h HRT had greater waterborne GSM levels at 3, 6, and 10 days post-stocking compared to 2.4 and 4.6-h HRT. A similar trend was generally reflected in salmon flesh. Residual GSM levels were successively higher in fillets on Day 6 from depuration systems with increasingly longer HRT. Geosmin levels were greatest in salmon flesh from the 11.3-h HRT treatment on Day 10, but fillet GSM between the 2.4 and 4.6-h HRT was similar. This research indicates that lowest residual GSM is achieved in water and Atlantic salmon flesh in depuration systems with increased flushing and shorter HRT, i.e., 2.4–4.6-h under conditions of this study. Selection of optimal flushing rate to remediate off-flavor from RAS-produced Atlantic salmon may also be dictated by water and energy use metrics and site-specific water availability among other factors.