Acid–base balance in sea bass (Dicentrarchus labrax L.) in relation to water oxygen concentration S Cecchini & A R Caputo Department of Sciences of Animal Production, University of Basilicata,Via Nazario Sauro, Potenza, Italy Correspondence: Dr S Cecchini, Department of Sciences of Animal Production, University of Basilicata,Via Nazario Sauro 85,85100 Potenza, Italy. E-mail: cecchini@unibas.it Abstract The in£uence of water oxygen concentration on the acid^base balance of sea bass was evaluated. Fish weighing 200^250 g were cultured under di¡erent dissolved oxygen concentrations of 64%, 97%, 150% and 250% saturation (92.7, 140.5, 217.5 and 362.7 mmHg respectively) under mild hypoxia, nor- moxia, mild hyperoxia and high hyperoxia condi- tions. The results showed that high hyperoxia and mild hypoxia conditions modi¢ed some blood para- meters signi¢cantly when compared with ¢sh held under the normoxia condition, while no di¡erences were shown with respect to the acid^base balance of ¢sh cultured under normoxia and mild hyperoxia conditions. This testi¢es that the mild hyperoxia con- dition does not produce physiological disturbances in the acid^base status of sea bass and it could be con- sidered a favourable condition in sea bass land-based farming, mainly in comparison with the mild hypox- ia condition, responsible for other physiological problems. Keywords: acid^base balance, sea bass, water oxygenation Introduction In ¢sh, unlike terrestrial animals, the breathing rate is primarily based on oxygen availability (Gilmour 1998). This typical response is due to the unsettled oxygen partial pressure in water. With respect to acid^base equilibrium, it is obtained by ¢sh exchan- ging, with the surrounding environment HCO 3 À /Cl À and H 1 /Na 1 in equal amounts in order to maintain electroneutrality (Heisler 1986). With respect to the acid^base balance of ¢sh cultured under di¡erent dissolved oxygen (DO) concentrations, more informa- tion is available for ¢sh held under deep short-term hypoxia (Thomas & Hughes 1982a, b;Thomas, Fievet & Motais 1986; Maxime, Pichavant, Boeuf & Nonnotte 2000) and severe short-term hyperoxia (Wood & Jack- son 1980; Goss & Wood 1990; Takeda 1990) con- ditions. According to Thomas & Hughes (1982a), a deep hypoxia condition (40 mmHg) causes respira- tory and metabolic acidosis in rainbow trout (Oncor- hynchus mykiss Walbaum), with an increase in lactate levels, while a moderate hypoxia condition (60 mmHg) only provokes an increase in the ventila- tory frequency, with respiratory alkalosis during the following 24 h. Severe environmental hyperoxia in- duces respiratory acidosis due to hypoventilation and hypoperfusion of the respiratory apparatus (Goss & Wood 1990), causing acid^base balance distur- bances similar to those observed for short- and long- term environmental hypercapnia (Claiborne & Heisler 1986; Perry, Malone & Ewing 1987; Fivestal, Olsen, Kloften, Ski & Stefansson 1999; Cecchini, Saroglia, Caricato,Terova & Sileo 2001). Nevertheless, major information being available on the e¡ects of severe hypoxia and/or elevated hyperox- ia, little information may be found regarding the e¡ects of long-term exposure of ¢sh to DO concen- trations simulating conditions in which most aqua- culture farms are operative. In Mediterranean land-based farming, DO avail- ability represents one of the more important limiting factors, mainly during the summer season, when the high temperature reduces the oxygen solubility. In land-based rearing conditions where paddling aeration is applied, DO concentrations are usually 60^80% of the saturation value, whereas on farms Aquaculture Research, 2003, 34, 1069^1073 r 2003 Blackwell Publishing Ltd 1069