Monitoring of oxygen fluctuations in seabass cages (Dicentrarchus labrax L.) in a commercial fish farm in Greece

Dissolved oxygen and temperature were monitored in cages stocked seabass at a commercial fish farm in Greece during summer and autumn. During the first part of the study, October–November 2012, current velocity monitoring outside the cages supported the sampling program. The latter sampling took place during May–July 2014. Including both periods, temperature fluctuated within the range 18–26°C while the current velocity, dominated by the tidal flow, varied from 0 to 28 cm/s in autumn. During sampling, seabass of 400–550 g individual weight was stocked at a density of 10–15 kg/m3. Within the cages, dissolved oxygen (DO) saturation decreased by 10% to 50% compared with outside levels. In-cage oxygen concentrations seemed to be affected by current speed, temperature and diurnal fluctuations due to the fish stock's activity and the day-night rhythm of algae. Hypoxia within the range 40%–70% of oxygen saturation was shown in several cases, despite the fact that the feeding intensity was quite low in the farm. The diurnal pattern of DO saturation showed that minimum levels coincided with feeding of the fish, while photosynthesis played a minor role in this case. This level of DO saturation although does not result in any mortalities may have other severe implications, especially at high temperature, such as lower feed utilization, increased metabolic expenses due to osmo-respiratory compromise, increased stress level, lower disease resistance and diminished fish welfare.