An 84-day experiment assessed the combined effects of two fresh water quality levels (H: 1.71 ± 0.15 mg O2l −1 and 0.28 ± 0.01 mg l −1 total ammonia nitrogen (T-AN), L: 5.15 ± 0.07 mg O2l −1 and 0.54 ± 0.01 mg T-AN l −1 ) and 3 stabilized stocking densities: 24.8 ± 0.2, 74.2 ± 0.5 and 120.0 ± 0.9 kg m −3 ) on rainbow trout. Fish were fed using demand feeders with rewards proportional to stocking density. Mass increase was significantly affected by water quality and stocking density, being highest in H water and the lowest at 120 kg m −3 . There was no significant difference in final weight between 25 and 74 kg m −3 , but at 120 kg m −3 it was 27% and 19% lower in H and L water respectively than at 25 kg m −3 . Feed intake (FI) from day 0-85 was significantly affected by water quality, 1.5% in H compared to 1.1-1.0 in L, but there were no significant differences in apparent feed conversion (AFC). FI was not significantly affected by stocking density but AFC was impaired, it increased with stocking density. Marked changes in fish morphology and composition were related to water quality: Condition K factor and fillet fat content were significantly higher in H than in L groups. Dorsal and pectoral fin condition was affected by stocking density and water quality: fins were significantly longer and less eroded in L groups and at low stocking density. Physiological measures were within the usual ranges, but differed between treatments. Changes in plasma osmolarity, hydromineral balance (Na + ) and acid base balance (HCO − ) showed that fish were more affected by water quality than by stocking density. There was no sign of acute stress in acclimated fish as cortisol and glycemia were similar under all experimental conditions. This study highlights the importance of water quality and feeding conditions when considering the effects of stocking density on fish welfare.

The chronic effects of exposing sea bass (average initial weight 100 g) to ammonia in water at 22 degreesC were first evaluated over a 61-day period (period 1, P1) during which nine different groups were submitted to nine ambient ammonia levels ranging from 0.014 to 0.493 mg 1(-1) NH3-N (0.53-16.11 mg 1(-1) total ammonia nitrogen (TA-N)) and fed using self-feeders. At the end of P1, the fish were starved for 10 days (P2). Their recovery capacity was tested over 43 days (P3) after which the exogenous ammonia supply was stopped in all treatments and the fish were allowed to feed. After 20 days of exposure a highly significant effect of ammonia was evident from the decrease in feeding activity, voluntary feed intake (VFI) and specific growth rate (SGR), and the increase in the feed conversion ratio (FCR). Ammonia exposure had no effect on circadian feeding rhythm or hourly actuation profiles. At the end of PI, the fish seemed to have adapted to all ambient ammonia concentrations tested since feeding and growth parameters were independent of ammonia levels. But they were unable to compensate for growth losses. Physiological adjustments were observed: plasma TA-N concentrations were positively related to ambient TA-N while there was no major disturbance in plasma urea. Plasma tri-iodo-thyronine concentrations were affected by ambient ammonia concentrations and there were no significant changes in hydromineral balance. During P2, oxygen consumption and urea excretion did appear to have been affected by ambient ammonia. When the exogenous supply of ammonia was stopped (M), fish exhibited hyperphagia and compensatory growth. In fish previously exposed to the highest ammonia levels, SGR and VFI were highest, and their FCR was improved. At the end of the experiment the final average weights were similar in all of the treatments (range 337-396 g). Depending on the concentrations used, ammonia exposure may enhance subsequent fish appetite and growth rate and have a similar effect on growth performances as restricting feeding level. Within the range tested, no detrimental effect of ammonia on the metabolic capacity of the fish, measured by oxygen consumption and urea excretion, or on their physiological status was recorded, and the fish had a good recovery capacity. In the conditions of the experiment, the non-observable effect concentration (NOEC) was 6 mg 1(-1). (C) 2003 Elsevier SAS and Ifremer/MD/Inra/Cemagref. All rights reserved.

Turbot juveniles were exposed to four ammonia concentrations [0·17 (L), 0·34 (M), 0·73 (MH) and 0·88 (H) mg l−1 NH3‐N] for different exposure durations (28 days minimum to 84 days). Their physiological status and growth performances were compared to a control group [0·004 (C) mg l−1 NH3‐N]. No growth was observed in the H group, and by day 57, mass increase in the MH group was only 15% of that in group C. During the first month growth in the L group was similar to that in control group while it was lower (33%) in the M group; afterwards the L and M groups had a similar growth (half that of controls). Accumulation of total ammonia nitrogen (TA‐N) in plasma was dependent on ambient ammonia concentrations. Plasma urea levels in ammonia‐exposed fish were lower, similar or greater than in controls (depending on ammonia concentration or exposure duration). Osmolarity, Cl– and Na+ plasma concentrations were stable in the L and M groups. The increases in Na+, Cl–, K+ and total Ca concentrations observed by the end of the experiment in the H and MH groups suggest that fish failed to adapt. There was an initial rise in plasma cortisol in all ammonia‐exposed groups followed by a return to basal level (1·7–4 ng ml−1) in the L and M groups. In group MH, plasma cortisol peaked at 42 ng ml−1 by day 14, and after a decline at c. 1 month (14 ng ml−1), it rose again.

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.