Sea bass (Dicentrarchus labrax) (135 ± 4 g) were reared under tank-based recirculating aquaculture system for a 63-day period at four densities: 10, 40, 70, 100 kg m −3. Fish performance, stress indicators (plasma cortisol, proteonemia plus other blood parameters-Na + , K + , glucose, pH, total CO 2 −) and water quality were monitored. At the end of the 63-day period, resistance to infection was also studied by a nodavirus challenge. A 25-day test was performed on fish from two extreme densities (10 and 100 kg m 3) and one intermediate density (40 kg m 3). With regards to the different density treatments, there was no significant difference between the daily feed intake (DFI) and the specific growth rate (SGR) up to a density of 70 kg m −3. No significant difference was found between treatments concerning the feed conversion ratio (FCR) and the mortality rate. No density effect was observed on the fish stress level (plasma cortisol) or on sensitivity to the nodavirus challenge. Under these experimental rearing conditions, the density above 70 kg m −3 has an impact on growth performance (DFI and SGR) indicators and also some blood parameters (CO 2) at the highest density tested (100 kg m −3). This study suggests that a density up to 70 kg m −3 has no influence on sea bass performance and welfare. At 100 kg m −3 , average specific growth rate was decreased by 14% without welfare deterioration according to the welfare indicators monitored.

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-Reared sea bass were submitted to six stressful situations: hyperoxia with or without hypercapnia, increased stocking density in an open flow or recirculating system, transfer to another tank and nodavirus injection. The potentially negative impact of these factors on the lives of sea bass was investigated by measuring 9 water parameters and 19 fish parameters including total serum protein (TSP). TSP has already been used to evaluate stress. The present study investigates data of previous and new experiments, concentrating on the potential use of TSP as a routine indicator to assess welfare in sea bass reared on fish farms. In the current experiment, oxygen and carbon dioxide were seen to affect levels of TSP, but alterations were too erratic to enable proper comparison, probably because they are normal components of the fish environment and become toxic only by dose increase. TSP decreased when stocking density increased. After transfer to another tank, TSP decreased to 14% for three weeks and then increased during the fourth week through compensative overproduction, before returning to normal levels after 2 months. The results confirmed that transfer is an important stress factor for fish, with cumulative effects for successive transfers. TSP alteration of nodavirus-injected fish depends on the type of symptoms, which can be divided into 3 groups: (i) dying fish, in which TSP increased sharply due to over-production of protein involved in non-specific defences and inflammation, then decreased dramatically before death; (ii) whirling fish, a group that included both fish that later died, in which TSP decreased, and fish that subsequently survived, in which TSP increased due to development of specific immunity; and (iii) asymptomatic fish, in which TPS was similar to control levels and which were probably insensitive to nodavirus and/or had developed defences. In this paper, different mechanisms of TSP alteration are proposed and the interest of TSP as a field parameter is discussed. TSP is a non-destructive parameter that is robust, easy to measure everywhere and cheap, representing a suitable way of monitoring the overall welfare of fish by its regular increase. It can be used only as a "warning" of poor rearing conditions, however, and further investigations would be needed to identify the specific stress or health disorder.

The relationship between serum protein, immunoglobulin concentrations and protein molecular weight profile (PMWP) alterations of sea bass (131.3 ± 4.3 g) reared in sea water with sublethal concentrations of ammonia was studied over two periods totalling 116 days. During the exposure period (62 days) the first group (group 1) lived in sea water with 0.204 mg•L-1 unionized ammonia nitrogen equivalent to 12 % of the lethal concentration for 50 % of a population exposed for 96 h to ammonia (96-h LC 50), whilst the second group (group 2) lived in sea water with 0.340 mg-L-1 UIA-N equivalent to 20 % of the 96-h LC 50 of ammonia. Then, the two groups were left for a recovery period (54 days) in the same water as the control group (group 0). The determination of the total immunoglobulin (Ig) concentration was carried out by enzyme-linked immunosorbant assay (ELISA). Gel filtration columns were used for the serum PMWP. Serum Ig concentration of the exposed fish (group 2: 1.76 ± 0.43 mg•mL-1 ; group 1: 1.19 ± 0.33 mg•mL-1) was lower than the control group (3.39 ± 1.01 mg•mL-1) after 21 days of exposure period but this difference was reduced at the end of this period (day 62) and treated group Ig concentrations switched to higher than the control group after the recovery period (day 116, group 2: 9.75 ± 1.84 mg•mL-1 ; group 1: 7.50 ± 1.22 mg•mL-1 group 0: 6.38 ± 1.13 mg•mL-1). In fact, at the end of the experiment, the cumulative Ig production difference between fish exposed to ammonia and the control was less than 10 %. Although a similar evolution of the Ig serum occurred with protein concentration, the serum protein concentration deficit of group 1 was restored at the end of the exposure period (group 2: 46.49 ± 2.34 mg•mL-1; group 0: 46.74 ± 1.97 mg•mL-1) and the cumulative production during the experiment was not significantly different between group 1 and group O. However, this remained lower for group 2. During the exposure period, the PMWP of treated fish moved towards smaller molecular weight proteins. This alteration of the PMWP showed that the 0.2-kDa fraction increased, and another fraction appeared at the end of the exposure period. However, at the end of the recovery period, no difference was found between the PMWPs of control and treated fish. The alterations of the serum protein and Ig of fish reared, for a Iimited period (2 months), in water with sublethal concentrations of ammonia (20 % 96-h LC 50 or less) should disappear completely after a few months in normal rearing conditions. © 1999 Ifremer/CnrslInralIrdlCemagreflÉditions scientifiques et médicales Elsevier SAS Ammonia 1 sublethal toxicity 1 immunoglobulin 1 proteins 1 fish culture 1 Dicentrarchus labrax Résumé-Modifications des protéines et des immunoglobulines sériques chez le loup (Dicentrarchus IIlbrax) consécutives à une exposition chronique à l'ammoniaque. Les modifications des concentrations en immunoglobulines (Ig) et en protéines ainsi que celles des protéinogrammes sériques ont été suivies pendant 116 jours chez des loups méditerranéens de ...