Introduction
Fish reared in recirculating aquaculture systems often accumulate off-flavor compounds, such as geosmin (GSM) and 2-methylisoborneol (2-MIB), negatively affecting product quality. Traditional depuration with a constant supply of fresh water is effective but water-intensive. Moreover, this lengthy process is potentially suboptimal for fish welfare. The patented advanced oxidation process (eAOP®) technology (Exiton Clean, USA) can offer a sustainable alternative by rapidly degrading off-flavor molecules during depuration, reducing water use and shortening depuration time (Kropp et al., 2022). This study validates the safety and effectiveness of eAOP in sturgeon depuration for caviar production.
Materials and methods
A depuration experiment was conducted in six 1.3 m3 tanks: triplicate control tanks equipped with a circulation pump and oxygenation, and triplicate tanks equipped with a circulation pump coupled to an eAOP reactor and oxygenation. Each tank hosted 32 fish (50 kg m-3) of male Danube sturgeons (Acipenser gueldenstaedtii) of ~1.9 kg individual body weight. Fish were not fed during the trial. Water temperature was maintained at 12°C. Water quality parameters were monitored daily and unionized NH3 concentration was kept under 0.05 mg l-1 by daily water changes. A steady H2O2 concentration of 10 mg l-1 was maintained in the tanks with eAOP reactor to sustain the advanced oxidation reaction. Water samples were collected daily and GSM and 2-MIB concentrations were determined by online extraction on a specific adsorbent, followed by gas chromatography with flame ionization detector (GC-FID). At day 0 and after 0.5, 1, 2, 3 and 4 weeks of depuration, fillet tissue and gill arches were sampled from three fish per tank to determine the GSM and 2-MIB concentrations in fillet tissue, and to detect unlikely gill damage due to chronic exposure to H2O2 respectively. GSM and 2-MIB concentrations in fillet tissue were determined by pressurized liquid extraction (PLE), clean-up using aluminum oxide and gel permeation chromatography, followed by analysis by gas chromatography–tandem mass spectrometry (GC–MS²). Macroscopic and histopathological analysis of eight most pronounced alterations (epithelial hypertrophy, epithelial hyperplasia, mucous cell alterations, chloride cell alterations, epithelial lifting, necrosis, hyperemia, angiectasia, inflammation) of the gill filaments were performed by two experts according to Marinovic et al. (2021).
Results and discussion
GSM and 2-MIB concentrations in water samples decreased below 10 ppt after day 23 and day 14 respectively in tanks without eAOP reactor, due to the daily water changes. In tanks with eAOP reactor, this concentration was already obtained after two days (Figure 1). Preliminary results of fillet samples show that GSM and 2-MIB concentrations decrease faster in fillets from fish kept in tanks with eAOP reactor, demonstrating the effectiveness of the eAOP reactor. Macroscopic examination demonstrates that gill lesions were largely absent. Almost all sampled gill arches exhibited no lesions and only five of 90 sampled fish, all from tanks without eAOP reactor, exhibited very light lesions. No moderate or severe macroscopic abnormalities were observed. Also, histopathological analysis did not show a significantly increased histopathological index of the gill arches of fish kept in tanks with eAOP reactor, demonstrating the safety of the eAOP reactor.
Figure 1: GSM and 2-MIB concentrations in water samples