Effects of repeated hypoxic shocks on growth and metabolism of turbot juveniles

Turbot juveniles (45 g) were exposed for 41 d (17 C, 34parts per thousand salinity) to constant normoxic (100-100% air saturation, 100-100) or moderate hypoxic (75-75% air saturation, 75-75) conditions and to repeated hypoxic shocks (20% saturation for 1 h, 5 d per week) from normoxic (100-20% air saturation. 100-20) or moderate hypoxic (75-20% air saturation, 75-20) conditions. A normoxic group was feed restricted (100-FR). Mass increase of 100-100 and 75-75 groups fed to satiation was not significantly different. In comparison, it was significantly lower in the 100-20 and 75-20 groups (NS between the two hypoxic shocks groups). Intermediate results were obtained in the 100-100-FR group. The lowest mass increase under hypoxic shocks was explained by a significant decrease in both feed intake and food conversion efficiency (FCE). FCE was lower in the two hypoxic groups, but only the 75-20 group was significantly different from all the other groups. There was no sign of stress and no change in the physiological status of fish in any group. When challenged, pre-conditioning of turbot to regular hypoxic shocks extended survival time, slightly but significantly, for 50% of the population. It was 8 h longer in starved than in fed fish. When reared for 1 year in normoxic water, the growth rate of post-challenged survivors was dependent on pre-conditioning: day 0-375 specific growth rate was significantly higher in the two groups acclimated to repeated hypoxic shocks. In the second experiment, it was shown that exposure to 20% air saturation for 12 h led to major physiological changes within 4 h: a significant decrease in plasma total CO2 and increase in plasma lactate contributing in maintaining blood pH stable, and a significant increase in osmolarity and chloride concentration. When returned to normoxic water, the recovery capacity of the fish was high: plasma osmolarity and total CO2 returned to pre-exposure levels within 1 h. The results are discussed in terms of turbot capacity to cope with repeated hypoxic shocks and to acclimate. (C) 2003 Editions scientifiques et medicates Elsevier SAS and Ifremer/IRD/Inra/Cemagref. All rights reserved.