Accumulation and depuration of geosmin and 2-methylisoborneol in Japanese seabass (Lateolabrax japonicus) fed diets containing different dietary protein and lipid levels in a recirculating aquaculture system

Muddy flavour caused by microbially synthesized geosmin and 2-methylisoborneol (2-MIB) represents a quality challenge for production of fish in recirculating aquaculture systems (RAS). The current study was conducted to find out if the accumulation and depuration of geosmin and 2-MIB in Japanese seabass (Lateolabrax japonicus) raised in RAS were affected by dietary protein and lipid levels, and to find out if these components were equally distributed in lean and fatty fish muscle tissues. A group of 540 fish with an initial weight of 0.11 kg were fed six extruded diets in triplicate tanks for 15 weeks for accumulation. A 3*2 factorial design with three dietary protein levels (420, 450, and 490 g kg-1) and two dietary lipid levels (150 and 180 g kg-1) was adopted. Six fish per tank were randomly selected after accumulation. Two of these were slaughtered as initial samples in a depuration trial. The other 4 fish per tank had passive integrated transponder tags implanted. The tagged fish were pooled into a 22 m3 depuration tank and fasted for 10 days. Water from RAS were also sampled after the accumulation trial and the microbial structures were identified by high throughput sequencing. The results showed that actinobacteria accounted for 0.98% of total bacteria, which was sufficient to induce a high degree of muddy flavour in the fish. Fish fed the diets with lower protein level accumulated more lipid and had more muddy flavour components in dorsal tissues (the upper part of the fillet) than those fed the high protein diets. The contents of lipid, geosmin and 2-MIB in the ventral tissues (belly) were significantly higher than that in dorsal tissues. The concentration of geosmin was higher than that of 2-MIB in all tissues and showed a positive correlation between the concentrations of geosmin and 2-MIB. The depuration rate of both geosmin and 2-MIB was higher in the ventral tissue than that in the leaner dorsal tissue during the 10-day fasting. Overall, high lipid diet resulted in fatty fish which caused rapid accumulation of geosmin and 2-MIB in fish. The duration of depuration was not sufficient to produce acceptable quality of the fish. Thus, a depuration period exceeding 10 days proved necessary for removing muddy off-flavour from a fatty fish like Japanese seabass