Dissolved oxygen regimen (PO2) may affect osmo-respiratory compromise in European sea bass (Dicentrarchus labrax, L.)

Fundamentally, in land based Mediterranean aquaculture, two techniques are applied to supply water with oxygen: paddling water aeration and application of pure oxygen. The two oxygenation techniques result in quite different PO2 regimens and, consequently, different fish growth performance and gill morphology. Data exist showing a reduction in total respiratory surface (RSA) and increasing gas diffusion distance (GDD) in gills of sea bass (Dicentrarchus labrax, L.) farmed under elevated PO2 regimens. That such a modification might have an effect on the ion regulation has been defined elsewhere as osmorespiratory compromise. In this study, European sea bass previously acclimatized to two PO2 regimens, mild hypoxia and mild hyperoxia (70-80% and 130-140% of the saturation value, respectively), were challenged for 1 hour with hypo-osmotic plus manipulation stress in two separate trials. During the first trial, when only Na+ loss was determined, the ion efflux during the first 5 min resulted in a rate of 163.72±31 and 112.23±87 nmol g–1min–1 from hypoxia and hyperoxia sea bass groups, respectively, and, if sustained, would approach 15.3 and 11.2% per hour of the total body Na+, respectively. During the second trial, in which both Na+ and Cl– loss were determined, after 60 min the Na+ loss was shown to be 76.86±12 and 179.28±32 nmol g–1 min–1 for the fish previously acclimatized t hyperoxia and hypoxia regimens, respectively, whereas for Cl- this loss was 62.02±11 and 157.28±28 nmol g–1min–1, respectively. Our data are compatible with the hypothesis of an osmotic advantage of sea bass exposed to an elevated PO2 regimen, achievable with application of pure oxygen, instead of simple water aeration.o