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In recirculating aquaculture systems (RAS), off‐flavours accumulated in fish muscle tissue can be problematic in terms of consumer acceptance and the reputation of farmed fish products. Off‐flavours often give fish earthy, muddy, or other undesirable flavours. Typically, off‐flavours are removed during a depuration period in which fish are fasted and held in clean water. Unfortunately, this causes additional costs and delayed sales, while fish lose weight and show a decrease in lipid content. First, we studied fish growth in a partial RAS (PRAS) where the conditions are very similar to those in depuration with a water exchange rate of 4000 L kg−1 feed, compared to RAS with a 650‐L water kg−1 feed. Rainbow trout (Oncorhynchus mykiss) was reared in both systems. Our aim was to combine the benefits of a higher water exchange rate: the lack of need for biofilters and a lower accumulation of off‐flavours while obtaining stable rearing conditions. Additionally, we studied the effects of moderate feeding and H2O2 addition during depuration. The fish grew faster in a PRAS than in a RAS when fed ad libitum. Thirteen off‐flavour compounds were found in the fish flesh and 11 in the circulating water. The H2O2 addition led to decreased levels of off‐flavours in the tank water and in fish muscle. The results showed no significant differences in off‐flavours between the fed and not‐fed systems, showing that moderate feeding did not prevent a good depuration result. However, the lipid content and the overall fish weight were higher in the fed systems, which suggests more effective depuration. Increased depuration efficiency can be an important tool when considering ways to improve the profitability of production. [ABSTRACT FROM AUTHOR]

The aquaculture industry is growing fast but facing two major challenges: a shortage of suitable locations for growth and the need to reduce environmental impacts. One solution for both these challenges is inland production through recirculating aquaculture systems (RAS). The RAS technique is rather new, and several practical issues need to be solved. In this study, an experimental platform, consisting of ten individual RAS units, was built for small-scale testing of different RAS designs and operation methods, and two preliminary experiments were conducted. In the first experiment, the capability of different chemical additions (sodium nitrite, ammonium chloride and/or cane sugar) to fasten the startup of the nitrification bioreactor was tested. In addition, the suitability and reliability of an online water measurement system in monitoring nitrification process with was evaluated. We demonstrated that when using a combination of sodium nitrite and ammonium chloride in a concentration of 5 mg l −1 , nitrification started one week before than when using only ammonium chloride or a clean start with rainbow trout ( Oncorhynchus mykiss ). In the second experiment, the effect of different relative water renewal rates (RWR) on water quality, rainbow trout growth and feed conversion ratio (FCR) were examined at 16 °C. Based on the results, FCR increased when RWR went below 478 l kg −1 , and the specific growth rate decreased when RWR went below 514 l kg −1 . Furthermore, when RWR decreased, nitrate, nitrite and organic material accumulated in the circulating water. In conclusion, we showed using experimental RAS platform that online water quality monitoring is a useful tool in following the effect of different management practices. Furthermore, we demonstrated that chemical substrate additions provide the fastest biofilter startup, and that water management is still in the key role in defining the fish production in RAS. [ABSTRACT FROM AUTHOR]

Abstract: The disappearance of malachite green (MG) residues was determined in fry of rainbow trout (Oncorhynchus mykiss) after six repeated treatments of the eggs at the hatching stage with MG oxalate at exposure levels of 1, 3 and 6mg l−1 for 30min. Fry samples were taken from newly hatched fry (0days post-hatch, d.p.h.) and at regular time intervals at 16, 31, 43, 57 and 96d.p.h. The residues of MG and its major metabolite, leucomalachite green (LMG), were found to accumulate in the fry after MG treatments of eggs, with the highest residue levels being determined in the newly hatched fry. After exposures of 3mg l−1 MG, mean concentrations of 1170±106µg kg−1 and 276±38.6µg kg−1 (n =3) were found in fry for LMG and MG, respectively. However, the disappearance of residues occurred rapidly in the fry, such that by 43d.p.h. only low levels of LMG could be determined. To confirm the elimination of residues, determinations were made also in fry muscle at 96d.p.h. but no residues were detected. The residues of MG in fry were determined by liquid chromatography-tandem mass spectrometric (LC-MS/MS) analysis with a limit of detection (LOD) of 0.5µg kg−1 and a limit of quantification (LOQ) of 1.0µg kg−1. The accumulation as well as the elimination of residues correlated well with the level of exposure. During the study, the fry increased their weight, such that at the end of the study, their mean body weight was about 150 times greater than the mean body weight of the newly hatched fry. As the disappearance of residues occurred in conjunction with the growth of fry, the present results indicate that no residues of MG will remain in the fish intended for human consumption, if MG treatment takes place at the hatching stage under controlled conditions. [Copyright &y& Elsevier]