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Fish is increasingly produced in recirculating aquaculture systems (RAS), which can offer some production and environmental benefits. Unfortunately, there is a tendency to develop off-flavors in RAS, mainly caused by geosmin (GSM) and 2-methyl isoborneol (MIB) accumulating in the fish flesh from the circulating water. These off-flavors are objectionable to consumers, reducing the profitability of production and causing an unwanted reputation for this fish production sector. In this study, combinations of oxidative chemicals: ozone (O3), hydrogen peroxide (H2O2), and peracetic acid (PAA) were applied in a commercial, full-scale (1.3 M kg a−1) RAS rearing rainbow trout (Oncorhynchus mykiss). Inlet water from a nearby lake, circulating water, and depuration water were treated to increase their quality and enhance the depuration procedure. The results showed a decrease of 77% (MIB) and 93% (GSM) in the concentrations of off-flavors in circulating water after the treatments with oxidative chemicals. [ABSTRACT FROM AUTHOR]

One-summer-old rainbow trout (Oncorhynchus mykiss ) were exposed to continuous hyperoxia (173 ± 24%) and three hyperoxic/normoxic treatments for 14 days. Hepatic glutathione status as the indicator of oxidative stress, as well as classical stress indicators such as hemoglobin, hematocrit and plasma cortisol levels, were measured during normoxic, constantly hyperoxic and the following episodically hyperoxic oxygen treatment regimes: 12 h hyperoxia:12 h normoxia (12 HYP:12 NOR), 24 HYP:24 NOR and 48 HYP:24 NOR. Constant hyperoxia tended to shrink erythrocytes, but the 12 HYP:12 NOR treatment increased the number of erythrocytes and thus enhanced the oxygen carrying capacity of blood. Similarly, a trend toward an elevation in plasma cortisol concentrations was detected in 12 HYP:12 NOR treatment group. The finding that elevated hepatic glutathione (GSH) levels (P < 0.01), indicative of enhanced potential of the liver tissue to resist oxidative stress, coincided with elevated cortisol levels might suggest that in the 12 HYP:12 NOR treatment physiological processes were recruited to increase oxygen carrying capacity in blood and to elevate protection against oxyradicals. However, none of the episodic hyperoxia treatments or continuous hyperoxia caused mortality or resulted in better growth. These data indicate that continuous hyperoxia (173 ± 24%) and hyperoxic-normoxic treatments may be applied in intensive culture of rainbow trout provided that fish have at least 24 h in normoxia prior to the next bout of hyperoxia. Shorter recovery periods, like in a 12 HYP:12 NOR treatment, may result in the increased need of oxygen in tissues followed by an activation of glutathione dependent defence system against an increased oxygen load. [ABSTRACT FROM AUTHOR]