No abstract

A variety of factors affecting water quality in recirculating aquaculture systems (RAS) are associated with the occurrence of off-flavours. In this study, we report the impact of water quality on the bacterial diversity and the occurrence of the geosmin-synthesis gene (geoA) in two RAS units operated for 252 days. Unit 2 displayed a higher level of turbidity and phosphate, which affected the fresh water quality compared with unit 1. In the biofilter, nitrification is one of the major processes by which high water quality is maintained. The bacterial population observed in the unit 1 biofilter was more stable throughout the experiment, with a higher level of nitrifying bacteria compared with the unit 2 biofilter. Geosmin appeared in fish flesh after 84 days in unit 2, whereas it appeared in unit 1 after 168 days, but at a much lower level. The geoA gene was detected in both units, 28 days prior to the detection of geosmin in fish flesh. In addition, we detected sequences associated with Sorangium and Nannocystis (Myxococcales): members of these genera are known to produce geosmin. These sequences were observed at an earlier time in unit 2 and at a higher level than in unit 1. This study confirms the advantages of new molecular methods to understand the occurrence of geosmin production in RAS.

The presence of off-flavor compounds in fish represents a significant economic problem encountered in aquaculture production. The off-flavor compounds are due to the absorption of substances produced by a range of microorganisms. Currently, a number of strategies have been used to prevent or limit the growth of these microorganisms. Therefore, it is important to evaluate the effectiveness of strategies via monitoring the concentrations of off-flavor compounds in the recirculating aquaculture system. In vivo solid-phase microextraction (SPME), a rapid and simple sample preparation method, will allow monitoring the concentration of off-flavor compounds in live fish. In this research, geosmin and 2-methylisoborneol (2-MIB) produced by cyanobacteria and actinomycetes, which are the major sources for "earthy" and "muddy" flavors in fish, were selected as representatives. In order to accurately quantify these compounds in fish muscle, two kinetic calibration methods, on-fiber standardization and measurement using predetermined sampling rate, were used as quantification methods, which were both validated by traditional methods. The detection limit of in vivo SPME in fish muscle was 0.12 ng/g for geosmin and 0.21 ng/g for 2-MIB, which are both below the human sensory thresholds.

The aquaculture industry has long recognized the need to reduce phosphorus (P) waste outputs associated with environmental impact, and reduce off-flavor producing compounds, which can impact the quality of the fish product. This study was undertaken to investigate the effects of dietary high P (HP) and low P (LP) on growth, nutrient digestibility, P retention, and P loading as well as their correlation to the synthesis of geosmin-associated off-flavor in a recirculating aquaculture system of rainbow trout Oncorhynchus mykiss. The above diets were fed to quadruplicate tanks of rainbow trout (average mass ± SD: 127.4 ± 3.1 g) for 84 d. Results showed that the effects of the HP and LP diets on growth and P retention were not significantly different. While the apparent digestibility of P and other nutrients were higher in fish fed the LP diet, P waste outputs and geosmin levels in the fillets of fish were higher in fish fed the HP diet. Magnesium (Mg 2+ ), potassium (K + ) and zinc (Zn 2+ ) concentrations in tank water were significantly lower in fish fed the HP diet than the LP diet in most of the sampling events. Furthermore, the tank water geosmin concentration was not strongly proportionally correlated with tank water-soluble P concentration for both the LP and HP diets. There was a strong proportional linear relationship between the geosmin concentration in tank water and in trout fillet for both the LP and HP diets. Results suggest that off-flavor contents in fish fillets and water were related to the dietary P level and metabolic P waste outputs into the system, findings that have implications for the formulation of sustainable diets for rainbow trout.