Effects of hyperoxic water conditions under different total gas pressures in European sea bass (Dicentrarchus labrax)

Sea bass (Dicentrarchus labrax, mean start weight 82 g) were exposed to experimental conditions for a period of 84 days. Water supplies with three different levels of gas content (no. 1 — normbaric normoxic, no. 2 — hyperbaric hyperoxic and no. 3 — normbaric hyperoxic) were prepared in three separate header tanks and mixed to the required exposure levels prior to entering the fish tanks.Experimental conditions consisted of three levels of oxygen saturation measured in effluent water and four levels of ΔP. The hyperbaric hyperoxia condition in water no. 2 was obtained by dissolving oxygen under high pressure while the normbaric hyperoxic condition of water no. 3 was obtained by dissolving oxygen under low pressure. The experimental groups consisted of a control group (99% O2, ΔP − 150 mm Hg), two medium hyperoxia groups (mH1: 135% O2, ΔP −160 mm Hg and mH2: 139% O2, ΔP + 29 mm Hg) and one high hyperoxia group (hH: 220% O2, ΔP + 167 mm Hg). Each treatment was performed in triplicate tanks and water was supplied by gravity from the header tanks.As a consequence of the treatment significant increase in mortality rate was observed in the hH group, along with significant reduction in growth, significant increase in food conversion rate and observations of gas bubbles in and around the eyes, at the base of and on the dorsal and pectoral fins. Improved growth was found in both the medium hyperoxia groups, though only the mH1 treatment group showed significant (p < 0.001) improvement in growth compared to control (+ 5.8%). No statistical difference was found in the blood values Na, K and pH amongst the treatments. Fish from the different groups were challenged with betanodavirus at the end of the oxygen exposure period and reared for 20 days in rearing conditions similar to the control group. The gas bubbles observed in the hH group disappeared spontaneously as soon as fish were placed in control conditions and no significant mortality or morbidity differences were seen between groups following the virus challenge. Results from the exposure period indicate a detrimental effect of exposure to O2 levels above 220% at elevated total gas pressure (ΔP + 167 mm Hg), and a positive growth effect when exposed to mild hyperoxia (135–139% O2). Although exposure to elevated total gas pressure and O2 levels above 220% is not recommended, the quick recovery observed in the fish when exposed to normoxic conditions suggests a degree of adaptability to such conditions.